84 OUTLINES OF EVOLUTIONARY BIOLOGY 



as we shall see later on, it is the union of the nuclei which is 

 the really important part of the business, for in some cases (e.g., 

 ParamcBcium) the union of the two cell bodies is a merely 

 temporary affair, a necessary preliminary to an exchange and 

 subsequent union of nuclei. 



In such simple cases as that of Copromonas we see all the 

 essential features of the sexual process which occurs so constantly 

 throughout the animal and vegetable kingdoms. It is evident 

 that in itself conjugation is not a process of reproduction, for its 

 immediate result is to halve the total number of cells instead of 

 doubling it. It has in fact exactly the opposite effect to that of 

 cell-division. It is a process which appears to be necessary for 

 the rejuvenescence, at longer or shorter intervals, of exhausted 

 cells, whereby they are endowed with renewed powers of multi- 

 plication by ordinary cell-division. At the same time it forms 

 the starting point of all those remarkable structural modifications 

 of the organism, whether unicellular or multicellular, w r hich 

 accompany the evolution of sex. 



In Copromonas and in Hsematococcus, although there is a 

 true sexual process, there is apparently no sexual differentiation 

 at all ; there is no distinction between male and female gametes ; 

 the two conjugating cells are exactly alike, and the conjugation 

 is therefore said to be isogamous. In Copromonas, moreover, the 

 gametes or sexual cells are indistinguishable from the ordinary 

 individuals, every individual being at least a potential gamete. 

 Starting from such a case as this we find, even amongst the 

 unicellular plants and animals, every stage in the evolution of highly 

 specialized male and female gametes, differing widely from the 

 ordinary individuals and from each other. Conjugation will then 

 take place between two dissimilar gametes, and is said to be 

 anisogamous. 



The first hint, so to speak, of sexual differentiation is to 

 be observed in the behaviour of the conjugating gametes; it is 

 a physiological rather than a structural or morphological 

 phenomenon, and consists in the fact that one gamete is active 

 while the other remains comparatively passive. We shall find 

 that this distinction lies at the root of all sexual differentiation 

 throughout the animal and vegetable kingdoms. The more active 

 gamete is spoken of as male and the more passive as female. The 

 passivity of the female is intimately associated with and probably 

 to a large extent dependent upon the fact that it contains more 



