106 HEREDITY ANB SELECTION IN SOCIOLOGY 



Another point to be noticed in regard to the biological role of 

 ampliimixis is the extraordinarily minute adaptation of the two 

 conjugating cells to one another ; and the consummate art with 

 which Nature has effected these adaptations shows us once more 

 the very great importance of amphimixis as a biological factor. 

 In the simplest cases the conjugating cells are identical both in 

 size and in appearance ; subsequently a difierence in size begins 

 to manifest itself, and eventually there are always dimorphic 

 micro- and macro-gametes, or, as we term them, male cells and 

 female cells. The difference between these two kinds of cells 

 becomes more and more accentuated as we ascend the animal 

 hierarchy. The egg is bulky, with little locomotor power, sub- 

 stantially equipped with nutritive material. The sperm-cell, on 

 the other hand, is small, extremely mobile, devoid of nutritive 

 reserves, and consequently incapable of independent life ; its 

 utility for the egg lies in the fact that it introduces the centro- 

 some, the dynamic centre in the karyokinesis of the fertilised egg. 

 And then come the countless difierent adaptations of both the 

 egg and the spermatozoon in relation to different conditions of 

 life in difierent species — adaptations in respect to the organism 

 and in respect to each other ; and all these variations, which 

 involve the minutest details, show us better than anything else 

 the importance of amphimixis as a biological factor : for they 

 show us, to speak metaphorically, the pains which natural selec- 

 tion has taken in order to ensure its persistence. 



To sum up : The biological importance of amphimixis, for 

 those species which exhibit it, lies in its double efiect of pro- 

 moting constancy and increasing adaptability. The constancy of 

 species is secured by an equihbrium established between the 

 species and its environment ; and we have seen that amphimixis 

 tends, by eliminating variations diverging from the line of equili- 

 brium, to make normality coincident with constancy. On the 

 other hand, when the conditions change, and the species is con- 

 fronted with the alternative of readaptation or extinction, amphi- 



