ORIGIN OF SEX 29 



case produce two kinds of sperm-cells, one possess- 

 ing, the other lacking this special character. A germ- 

 cell of the female, fertilized by a sperm-cell of the 

 first kind would develop into a male, fertilized by a 

 sperm-cell of the second kind, would develop into 

 a female. If sperm-cells of both kinds were pro- 

 duced in approximately equal numbers, a germ- 

 cell would have an even chance of being fertilized 

 by either one or the other ; and an explanation is 

 forthcoming for the fact that, on an average, 

 males and females are born in about equal 

 numbers. It may be conjectured that sexual 

 generation, by imparting a shock to the life sub- 

 stance, maintains its essential instability and 

 prevents it from settling down into such a state 

 of equilibrium as would come about from undis- 

 turbed habit. However this may be, one point 

 is clear that sexual generation modifies indivi- 

 duality, and contributes very potently to the 

 changefulness of Life. 



We pass, now, to the consideration of still 

 greater changes transformations so wonderful 

 that until comparatively recent times their action 

 was hardly suspected. Individuals may change 

 and die ; but, so far as we can judge by ordinary 

 observation, there is no alteration in the type of 

 the species to which they belong. But, unless we 

 reject the doctrine of evolution, we must believe 

 that the type is subject to like changes with the 

 individual, and that, just as individual plants and 

 animals reach maturity through a number of em- 

 bryonic stages of advancing complexity, so have all 

 the species now existing worked their way up from 

 the bottom of the animal and vegetable kingdoms 

 by passing through metamorphoses, each of 

 which at one time, in bygone ages, stood for a 



