Darwin has given an admirably clear statement of his opinion as to the causes of variability in the opening paragraph of his chapter dealing with this topic in his “Animals and Plants.” Some authors, he says, “look at variability as a necessary contingent on reproduction, and as much an original law as growth or inheritance. Others have of late encouraged, perhaps unintentionally, this view by speaking of inheritance and variability as equal and antagonistic principles. Pallas maintained, and he has had some followers, that variability depends exclusively on the crossing of primordially distinct forms. Other authors attribute variability to an excess of food, and with animals, to an excess relatively to the amount of exercise taken, or again, to the effects of a more genial climate. That these causes are all effective is highly probable. But we must, I think, take a broader view, and conclude that organic beings, when subjected during several generations to any change whatever in their condition, tend to vary; the kind of variation which ensues depending in most cases in a far higher degree on the nature of the constitution of the being, than on the nature of the changed conditions.”
Most naturalists will agree, in all probability, with this conclusion of Darwin’s. The examples cited in the preceding pages have shown that there are several ways in which the organisms may respond to the environment. In some cases it appears to affect all the individuals in the same way; in other cases it appears to cause them to fluctuate in many directions; and in still other cases, without any recognizable change in the external conditions, new forms may suddenly appear, often of a perfectly definite type, that depart widely from the parent form.
For the theory of evolution it is a point of the first importance to determine which of these modes of variation has supplied the basis for evolution. Moreover, we are here especially concerned with the question of how adaptive variations arise. Without attempting to decide for the present between these different kinds of variability, let us examine certain cases in which an immediate and adaptive response to the environment has been described as taking place.
Responsive Changes in the Organism that adapt it to the New Environment
There is some experimental evidence showing that sometimes organisms respond directly and adaptively to certain changes in the environment. Few as the facts are, they require very careful consideration in our present examination. The most striking, perhaps, is the acclimatization to different temperatures. It has been found that while few active organisms can withstand a temperature over 45 degrees C., and that for very many 40 degrees is a fatal point, yet, on the other hand, there are organisms that live in certain hot springs where the temperature is very high. Thus, to give a few examples, there are some of the lower plants, nostocs and protococcus forms, that live in the geysers of California at a temperature of 93 degrees C., or nearly that of boiling water. Leptothrix is found in the Carlsbad springs, that have a temperature of 44 to 54 degrees. Oscillaria have been found in the Yellowstone Park in water between 54 and 68 degrees, and in the hot springs in the Philippines at 71 degrees, and on Ischia at 85 degrees, and in Iceland at 98 degrees.
It is probable from recent observations of Setchel that most of the temperatures are too high, since he finds that the water at the edge of hot springs is many degrees lower than that in the middle parts.
The snail, Physa acuta, has been found in France living at a temperature of 35 to 36 degrees; another snail, Paludina, at Abano, Padua, at 50 degrees. Rotifers have been found at Carlsbad at 45 to 54 degrees; Anguillidæ at Ischia at 81 degrees; Cypris balnearia, a crustacean at Hammam-Meckhoutin, at 81 degrees; frogs at the baths of “Pise” at 38 degrees.
Now, there can be little doubt that these forms have had ancestors that were like the other members of the group, and would have been killed had they been put at once into water of these high temperatures, therefore it seems highly probable that these forms have become specially adapted to live in these warm waters. It is, therefore, interesting to find that it has been possible to acclimatize animals experimentally to a temperature much above that which would be fatal to them if subjected directly to it. Dutrochet (in 1817) found that if the plant, nitella, was put into water at 27 degrees, the currents in the protoplasm were stopped, but soon began again. If put now into water at 34 degrees they again stopped moving, but in a quarter of an hour began once more. If then put into water at 40 degrees the currents again slowed down, but began again later.
Dallinger (in 1880) made a most remarkable series of experiments on flagellate protozoans. He kept them in a warm oven, beginning at first at a temperature of 16.6 degrees C. “He employed the first four months in raising the temperature 5.5 degrees. This, however, was not necessary, since the rise to 21 degrees can be made rapidly, but for success in higher temperatures it is best to proceed slowly from the beginning. When the temperature had been raised to 23 degrees, the organisms began dying, but soon ceased, and after two months the temperature was raised half a degree more, and eventually to 25.5 degrees. Here the organisms began to succumb again, and it was necessary repeatedly to lower the temperature slightly, and then to advance it to 25.5 degrees, until, after several weeks, unfavorable appearances ceased. For eight months the temperature could not be raised from this stationary point a quarter of a degree without unfavorable appearances. During several years, proceeding by slow stages, Dallinger succeeded in raising the organisms up to a temperature of 70 degrees C., at which the experiment was ended by an accident.”[[27]]
[27]. Quoted from Davenport’s “Experimental Morphology.”