194 NATURAL SCIENCE. Sept.. 



we can distinguish sexual reproduction — that is, reproduction after 

 conjugation — from reproduction in which no conjugation takes place, 

 or asexual reproduction. 



For an example of the latter we have not to go far, for the 

 microspores of Ulothrix, if they fail to meet and coalesce with other 

 microspores, may develop by themselves, gradually growing into adult 

 specimens. 



It will be observed from the foregoing that in a study of sexual 

 reproduction we have two important and quite distinct problems to 

 discuss, problems which have unfortunately too often been treated as 

 if they were one. In the first place, we may concentrate the attention 

 upon the act of conjugation, and enquire as to its utility ; and in the 

 second place, we may seek for an explanation of how it is that in the 

 vast majority of cases the conjugating forms are dissimilar to each 

 other, and may therefore be termed male and female. It will be 

 convenient to study the second problem — that of sexual dimorphism 

 — first, chiefly because its solution may clear the way for the first and 

 by far the most obscure question, " why should organisms conjugate 

 at all ? " 



Sexuai, Dimorphism. 



There can be little doubt that the earliest form of conjugation 

 was between forms similar to each other and exhibiting no sexual 

 difference, and that sexual dimorphism arose subsequently. This is 

 believed because in existing organisms it is only in very simple and 

 primitive types that we find the absence of sexual differences, and 

 these differences become most pronounced in more highly organised 

 and later types. 



If we enquire why dimorphism arose, v/e naturally ask ourselves 

 the question, what can have been and what are its special advan- 

 t iges ? To show the existence of an advantage, will, by the light of 

 Darwin's work, give at any rate a complete and reasonable hypothesis 

 for the development of dimorphism. Accepting the fact that living 

 organisms constantly tend to vary in an almost infinite number of 

 ways, we can readily understand how, at one time or another in the 

 history of a species, individuals might be met with whose reproductive 

 cells differed considerably from each other, say in size. If now it can 

 be shown that, for purposes of conjugation, or for anything else, 

 this variation had a distinct advantage over ordinary members of the 

 species whose reproductive cells were all of the same size, then we 

 may rely on natural selection for the perpetuation of this fortunate 

 variety. While the gift of a purse might be of advantage to a beggar 

 in our streets, it would little avail the traveller dying of thirst in the 

 Sahara : a possession, a power, a property may be of advantage to 

 one and not to another ; for this point will be determined by their 

 conditions of life. Probably sexual dimorphism would not be of 

 advantage to those lower forms which do not possess it ; otherwise it 

 is difficult to imagine its non-occurrence and non-preservation through 



