6. Variations in the time at which reproduction commences.
7. Variations in the duration and amount of parental protection and fosterage.
8. Variations in the period at which secondary sexual characters and the maximum efficiency of the several organs is reached.
It is impossible here to discuss these modes of variation seriatim. I shall therefore content myself with but a few remarks on the importance of protection and fosterage. It is not too much to say that, without fosterage and protection, the higher forms of evolution would be impossible. If you are to have a highly evolved form, you must allow time for its evolution from the egg; and that development may go on without let or hindrance, you must supply the organism with food and lighten the labour of self-defence. Most of the higher organisms are slow in coming to maturity, passing through stages when they are helpless and, if left to themselves, would inevitably fall a prey to enemies.
In those animals in which the system of fosterage and protection has not been developed a great number of fertilized ova are produced, only a few of which come to maturity. It might be suggested that this is surely an advantage, since the greater the number produced the greater the chances of favourable variations taking place. But it has before been pointed out that these great numbers are decimated, and more than decimated, not by elimination, but by indiscriminate destruction; embryos, good, bad, and indifferent, being alike gobbled up by those who had learnt the secret of fostering their young. The alternative has been between producing great numbers[DN] of embryos which soon fend for themselves, and a few young who are adequately provided for during development. And the latter have proved the winners in life's race. If we compare two flat-fishes belonging to very different groups, the contrast here indicated will be readily seen. The skate is a member of the shark tribe, flattened symmetrically from above downwards. It lays, perhaps, eighty to a hundred eggs. Each of these is large, and has a rich supply of nutritive food-yolk. Each is also protected by a horny case with pointed corners—the so-called sea-purse of seaside visitors. These are committed by the skate to the deep, and are not further cared for. But the abundant supply of food-yolk gives the little skate which emerges a good start in life. On the other hand, the turbot, one of the bony fishes, flattened from side to side with an asymmetrical head, lays several millions of eggs, which float freely in the open sea. These are minute and glassy, and not more than one-thirtieth of an inch in diameter. When the fishes are hatched, they are not more than about one-fifth of an inch in length. The slender stock of food-yolk is soon used up, and henceforth the little turbot (at present more like a stump-nosed eel than a turbot) has to get its own living. Hundreds of thousands of them are eaten by other fishes.
Or, if we compare such different vertebrates as a frog, a sparrow, and a mouse, we find that the frog produces a considerable number of fertilized ova, though few in comparison with the turbot, each provided with a small store of food-yolk. The tiny tadpoles very soon have to obtain their own food and run all the risks of destruction. Few survive. The sparrow lays a few eggs; but each is supplied with a large store of food-yolk, sufficient to meet its developmental needs until, under the fostering influence of maternal warmth, it is hatched. Even on emerging from the eggs, the callow fledglings enjoy for a while parental protection and fosterage, and, when sent forth into the world, are very fairly equipped for life's struggle. The mouse produces minute eggs with little or no food-yolk; but they undergo development within the womb of the mother, and are supplied with nutrient fluids elaborated within the maternal organism. Even when born, they are cherished for a while and supplied with food-milk by the mother.
The higher stages of this process involve a mental element, and are developed under the auspices of intelligence or instinct. But the lower stages, the supply of food-yolk and intra-uterine protection, are purely organic. A hen cannot by instinctive or intelligent forethought increase the amount of food-yolk stored up in the ovum, any more than the lily, which, by an analogous process, stores up in its bulb during one year material for the best part of next year's growth, can increase this store by a mental process.
It cannot therefore be questioned that variations in the amount of capital with which an embryo is provided in generation would very materially affect its chances of escaping elimination by physical circumstances, by enemies, and by competition.
Nor can it be questioned that variations in the time occupied in reaching maturity would, other things equal, not a little affect the chances of success of an organism in the competition of life. Hence we have the phenomena of what may be termed acceleration and retardation in development. These terms have, however, been used by American zoologists, notably Professors Hyatt and Cope, in a somewhat different and wider sense; for they include not merely time-changes, but also the loss of old characters or the acquisition of new characters. "It is evident," says Professor Cope, "that the animal which adds something to its structure which its parents did not possess has grown more than they; while that which does not attain to all the characteristics of its ancestors has grown less than they." "If the embryonic form be the parent, the advanced descendant is produced by an increased rate of growth, which phenomenon is called 'acceleration'; but if the embryonic type be the offspring, then its failure to attain the condition of the parent is due to the supervention of a slower rate of growth; to this phenomenon the term 'retardation' is applied." "I believe that this is the simplest mode of stating and explaining the law of variation: that some forms acquire something which their parents did not possess; and that those which acquire something additional have to pass through more numerous stages than their ancestors; and those which lose something pass through fewer stages than their ancestors; and these processes are expressed by the terms 'acceleration' and 'retardation.'"[DO]
It is clear, however, that we have here something more than acceleration and retardation of development in the ordinary sense of these words. It would be, therefore, more convenient to use the term "acceleration" for the condensation of the same series of developmental changes into a shorter period of time; "retardation" for the lengthening of the period in which the same series of changes are effected; and "arrested development" for those cases in which the young are born in an immature or embryonic condition. Whether there is any distinct tendency, worthy of formulation as a law, for organisms to acquire, as a result of protracted embryonic development, definite characteristics which their ancestors did not possess, I think very questionable. If so, this will fall under the head of the origin of variations.