160 THE PROTOZOA 



drawn between intrinsic differences, not necessarily visible, and 

 structural or other differences which are more or less obvious. 

 The fact that gametes and pronuclei tend to unite proves that in 

 all cases there must be intrinsic differences between them which 

 stimulate them to do so ; in this sense, at least, we may endorse 

 fully the dictum of Hertwig, that " fertilization depends on a 

 fusion of sexually-differentiated cell-nuclei." On the other hand, 

 gametes of opposite sexes exhibit every possible condition from 

 complete similarity in structure and appearance to the greatest 

 possible contrast in every feature of their organization. There 

 can be no doubt that visible differentiation of the gametes is 

 largely, if not entirely, an adaptation to the functions that they 

 have to perform ; and this conclusion is by no means weakened 

 by the fact that there are many cases of isogamy which are un- 

 doubtedly secondary, in which a more primitive and phylogeneti- 

 cally older structural differentiation has gradually become annulled, 

 under circumstances in which adaptive differences in the gametes 

 are no longer necessary as, for example, in gregarines (p. 173). 



In Metazoa it is generally recognized that the two pronuclei 

 that undergo fusion are perfectly equivalent,* and that the dif- 

 ferences seen between them in the gametes are temporary and, in 

 the case of the spermatozoon, an adaptation to circumstances ; here 

 the real differentiation of the gametes affects only cytoplasmic 

 characters. In Protozoa, on the other hand, the conjugating 

 pronuclei often exhibit differences of structure when the cells 

 themselves appear perfectly similar. In the Infusoria, for instance, 

 differences have been noted between the migratory and stationary 

 pronuclei ; how far these differences may be correlated directly with 

 the differences in their activities must remain an open question. 



In the foregoing paragraphs we have set forth and discussed 

 some of the attempts that have been made to solve the problem 

 of sex. It cannot be said that a perfectly satisfactory solution 

 has been attained, but at least certain conditions of the problem 

 may be laid down. In the first place, no theory of sex is satis- 

 factory which does not explain why the union of two cells should 

 be necessary in syngamy. In the second place a teleological inter- 

 pretation, such as amphimixis, can only state a secondary con- 

 sequence, not a primary cause, of sexual union ; but such a 

 consequence may suffice to explain the retention and persistence 

 of sexual phenomena after the conditions have ceased to exist under 

 which they came into existence. 



In the simplest Protista of the bacterial grade, it may be supposed, 

 either that the living matter is not differentiated into localized 

 substances having distinct physiological qualities, or that in such 



* Apart, that is to say, from the much -discussed question of the supernumerary 

 chromosome. 



