Evolution and Adaptation

by Thomas Hunt Morgan

A number of writers have objected to the general and often vague way in which Darwin makes use of this phrase; but it does not seem to me that this is a serious objection, provided we are on our guard as to what the outcome will be in each case. In each instance we must consider the question on its own merits, and if it is found convenient to have a sufficiently general and non-committal term, such as the “struggle for existence,” to include all cases, I see no serious objection to the use of such an expression, although it is true the outcome has been that it has become a catchword, that is used too often by those who have no knowledge of its contents.

Were it not that each animal and plant gives birth, on an average, to more than two offspring, the species would soon become exterminated by accidents, etc. We find in some of the lower animals, and in some of the higher plants, that thousands and even millions of eggs are produced by a single individual in the course of its life. A single nematode may lay sixty million eggs, and a tapeworm one thousand million. A starfish may produce about thirty-nine million eggs, a salmon may contain fifteen thousand, and a large shad as many as one hundred thousand. The queen of a termite nest is said to lay eighty thousand eggs a day.

In the higher vertebrates the number of young is considerably less, but since the young stages are passed within the body of the parent, proportionately more of them reach maturity, so that even in man the population may be doubled in twenty-five years, and in the elephant, slowest breeder of all animals, Darwin has calculated that, if it begins breeding when about thirty years old and goes on until ninety years, bringing forth six young in the interval, after 750 years there will be nearly nineteen million elephants alive which have descended from the first pair.

Obviously, then, if all the descendants of all the individuals of a species were to remain alive, the world would be over-crowded in a very short time, and the want of room would in itself lead to the destruction of countless individuals, if for no other reason than lack of food. We can easily carry out on a small scale an experiment that shows how the overstocking, resulting from favorable conditions, comes about, and how it checks itself. If we make a meat broth suitable for the life of a particular bacterium, and sow in the broth a very few individuals, we find in the course of several days the fluid swarming with the descendants of the original individuals. Thus it has been shown that, if we start with a few hundred bacteria, there will be five thousand after twenty-four hours, and twenty thousand, forty-eight hours later; and after four days they are beyond calculation.

Cohn found that a single bacterium produces two individuals in one hour, and four in two hours, and if they continue to multiply at this rate there will be produced at the end of three days 4,772 billions of descendants. If these are reduced to weight, they would weigh seventy-five hundred tons. Thus when the conditions are favorable, bacteria are able to increase at such an enormous rate that they could cover the surface of the earth in a very few days. The reason that they do not go on increasing at this rate is that they soon exhaust the food supply, and the rate of increase slows down, and will finally cease altogether. If the bacteria were dependent on a continuous supply of food, they would perish after the supply had been exhausted, so that the rapid rate of multiplication would serve only to bring the career of the organism to an untimely end. If the weaker individuals were to die first, the products of their disintegration might serve to nourish the stronger individuals; hunger coming on again, the next weakest might die; and the same process continuing, we might imagine that the bacteria were finally reduced to a single one which would then die in turn for lack of food. Like a starving shipload of men, reduced by hunger to cannibalism, the life of some and finally of the last individual might be prolonged in the hope of rescue, but if this did not arrive, the last and perhaps the strongest individual would perish. But this is not what we find occurring in these lower organisms, for, as a rule, they gradually cease to increase when the food supply becomes lessened, and their activities slow down. Finally, when the food is gone, they pass into a resting stage, in which condition they can remain dormant for a long time, even for years. If they should again find themselves in favorable surroundings, they become active, and begin once more their round of multiplication. We cannot follow the individuals in such a culture of bacteria, but there is nothing to be seen that suggests a struggle for existence, if this idea conveys the impression of the destruction of certain individuals by competition with others. In fact, the results are in some respects exactly the reverse. Millions of individuals are present at the time when the food supply becomes exhausted, and they all pass into a protected resting stage.

The enormous rate of increase in this case finds its counterpart in higher animals when the food supply, or the absence of enemies, allows a species to multiply at its maximum rate of increase. The introduction of rabbits into Australia was followed by an enormous increase in a few years, and the introduction of the English sparrow into the United States has had a similar result. But in no country can such a process continue beyond a certain point, because, in the first place, the scarcity of food will begin to keep the birth-rate down, and in the second place, the increase in numbers may lead to an increase in the number of its enemies, or even induce other forms to feed on it. Crowding will also give an opportunity for the spread of disease, which again may check the increase. Sooner or later a sort of ever shifting balance will be reached for each species, and after this, if the conditions remain the same, the number of individuals will keep approximately constant.

Darwin admits that the “causes which check the natural tendency of each species to increase are most obscure.” “We know not exactly what the checks are even in a single instance.” This admission may well put us on our guard against a too ready acceptation of a theory in which the whole issue turns on just this very point, namely, the nature of the checks to increase. Darwin gives the following general cases to show what some of the checks to increase are. He states that eggs and very young animals and seeds suffer more than the adults; that “the amount of food for each species of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends largely on the destruction of the vermin.” “On the other hand, in some cases, as with the elephant, none are destroyed by beasts of prey; for even the tiger in India most rarely dares to attack a young elephant protected by its dam.” “Climate plays an important part in determining the average number of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks.” “The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same, or of distinct species which subsist on the same kind of food.”

We need not follow Darwin through his account of how complex are the relations of all animals and plants to one another in the struggle for existence, for, if true, it only goes to show more plainly how impossible it is to establish any safe scientific hypothesis, where the conditions are so complex and so impossible to estimate. To show that the young Scotch fir in an enclosed pasture is kept down by the browsing of the cattle, and in other parts of the world, Paraguay for instance, the number of cattle is determined by insects, and that the increase of these flies is probably habitually checked by other insects, leads to a bewilderingly complex set of conditions. We cannot do better than to quote Darwin’s conclusion: “Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies—then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Staffordshire, the insectivorous birds, and so onwards in ever increasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one organic being over another. Nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!”

The effect of the struggle for existence in determining the distribution of species is well illustrated in the following cases:—

“As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera. We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species. The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush. How frequently we hear of one species of rat taking the place of another species under the most different climates! In Russia the small Asiatic cockroach has everywhere driven before it its great congener. In Australia the imported hive-bee is rapidly exterminating the small, stingless native bee. One species of charlock has been known to supplant another species; and so in other cases. We can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life.”