380 THE EVOLUTION THEORY 



well, sperm-cells of quite peculiar form occur, as, for instance, in the 

 crayfish and its marine relatives, the crabs and the long-tailed 

 Decapods. In these cases the spermatozoa bear long and stiff thorn- 

 like processes, which, as in the sperm-cells of Moina, make them ad- 

 hesive, and, according to Brandes, render it possible for them to cling 

 among the bristles on the abdomen of the female until one of the 

 many eggs leaving the oviduct comes within reach. For among these 

 Crustacea there is no true copulation, but the masses of sperm-cells 

 are packed together into sperm-packets or ' spermatophores,' and are 

 affixed by the male near the opening of the oviduct, where they burst 

 and pour forth their contents between the appendages of the female. 



All these remarkable and widely divergent structures and arrange- 

 ments depend not upon chance or on the fantastic expression of a 

 ' formative power,' as an earlier generation was wont to phrase it ; they 

 are undoubtedly without exception adaptations to the most intimate 

 conditions of fertilization in each individual case. I lay particular 

 stress upon a recognition of this, because it permits us to infer with 

 certainty that even the variations of the single cell, if they are 

 sufficiently important for the species, may be controlled by natural 

 selection. It is obvious that the adaptations of the sex-cells must 

 depend not on histonal selection, but only upon personal selection, since 

 it is indifferent for the individual sperm-cells whether fertilization is 

 accomplished successfully or not, while it is by no means indifferent 

 for the species. The organism dies without descendants if its sperm- 

 cells do not fertilize, and the carrying on of the species must be left to 

 those of its fellows which produced sperm-cells which fertilize with 

 more certainty ; thus it is not the sperm-cells themselves, but the 

 individual organisms which are selected, and that in relation to the 

 quality of the sex-cells they produce. 



In contrast with the great diversity of form exhibited by the 

 spermatozoa, the differentiation of the ovum appears very uniform, at 

 least in regard to form and activity. The main form is spherical, but 

 it is subject to many variations in the way of elongation or flattening. 

 In the lower forms of life, as, for instance, among the sponges, and also 

 in the polyps and Medusae the egg-cells possess, until they are mature, 

 the locomotor capacity of unicellular organisms ; they creep about after 

 the manner of amcebse, and indeed, as I showed years ago, this 

 movement from place to place in many polyps is exactly regulated; 

 thus at a definite time they may leave the place where they originated 

 and may, for instance, creep from the outer layer of cells (ectoderm) 

 of the animal into the inner layer (endoderm) by boring through the 

 so-called 'supporting lamella,' then they may creep further in the 



