This establishment by survival of the fittest of reproductive processes adapted to variable conditions, is indirectly elucidated by the habits of salmon. As salmon thrive in the sea and fall out of condition in fresh water (having during their sea-life not exercised the art of catching fresh-water prey), the implication is that the species would profit if all individuals ran up the rivers just before spawning time in November. Why then do most of them run up during many preceding months? Contemplation of the difficulties which lie in the way to the spawning grounds, will, I think, suggest an explanation. There are falls to be leaped and shallow rapids to be ascended. These obstacles cannot be surmounted when the river is low. A fish which starts early in the season has more chances of getting up the falls and the rapids than one which starts later; and, out of condition as it will be, may spawn, though not well. On the other hand, one which starts in October, if floods occur appropriately, may reach the upper waters and then spawn to great advantage; but in the absence of adequate rains it may fail altogether to reach the spawning grounds. Hence the species profits by an irregularity of habits adapted to meet irregular contingencies.
I owe to Mr. (now Sir John) Lubbock an important confirmation of this view. After stating his belief that between Crustaceans and Insects there exists a physiological relation analogous to that which exists between water vertebrata and land-vertebrata, he pointed out to me that while among Insects there is a definite limit of growth, and an accompanying definite commencement of reproduction, among Crustaceans, where growth has no definite limit, there is no definite relation between the commencement of reproduction and the decrease or arrest of growth.
While this chapter is passing through the press, I learn from Mr. White Cooper, that not only are near sight, long sight, dull sight, and squinting, hereditary; but that a peculiarity of vision confined to one eye is frequently transmitted: re-appearing in the same eye in offspring.
An instance here occurs of the way in which those who are averse to a conclusion will assign the most flimsy reasons for rejecting it. Rather than admit that the eyes of these creatures living in darkness have disappeared from lack of use, some contend that such creatures would be liable to have their eyes injured by collisions with objects, and that therefore natural selection would favour those individuals in which the eyes had somewhat diminished and were least liable to injury: the implication being that the immunity from the inflammations due to injuries would be so important a factor in life as to cause survival. And this is argued in presence of the fact that one of the most conspicuous among these blind cave-animals is a cray-fish, and that the cray-fish in its natural habitat is in the habit of burrowing in the banks of rivers holes a foot or more deep, and has its eyes exposed to all those possible blows and frictions which the burrowing involves!
In addition to the numerous illustrations given by Mr. Sedgwick, here is one which Colonel A. T. Fraser published in Nature for Nov. 9, 1893, concerning two Hindoo dwarfs:—"In speech and intelligence the dwarfs were indistinguishable from ordinary natives of India. From an interrogation of one of them, it appeared that he belonged to a family all the male members of which have been dwarfs for several generations. They marry ordinary native girls, and the female children grow up like those of other people. The males, however, though they develop at the normal rate until they reach the age of six, then cease to grow, and become dwarfs."