1895. 1'HE ROLE OF SEX. 195 



variation and natural selection. Equally probable is it that sexual 

 dimorphism is or was of advantage to those forms which now possess it. 



If now we study the conditions of life, both of those which are 

 not, and those which are, sexually dimorphic, and if we find certain 

 conditions absent in one case and present in the other, we may 

 assume, at any rate provisionally, that these latter conditions deter- 

 mined sexual dimorphism by making it advantageous wherever it 

 should appear as a variety. 



If we study in detail some of the life-histories of the lower forms, 

 both those which are dimorphic and those which are not, we shall 

 find that wherever certain difficulties in the way of conjugation 

 appear, there dimorphism is also present. 



Those forms which, though conjugating, are not dimorphic, are 

 of the most simple structure, consisting of single cells, or small 

 •colonies of these. In all cases, so far as I can determine, conjugation 

 is most easy and effectual, for they are either (i) free swimming, 

 (2) free crawling, or (3) they are nearly opposed to each other. As 

 an example of a free swimming form we may take the Pavamcecia, 

 which are free swimming and active infusoria. When conjugating, 

 two individuals come in contact by their ventral surfaces, exchange 

 nuclear matter, separate, and then divide. Of three Pavamcecia — A, 

 B, and C — A may conjugate with B or with C, or B may conjugate 

 with C ; three possibilities, two only of which could, of course, take 

 place were a sexual difference present. As another example of a free 

 swimming form, Hetero)nita may be instanced. 



As an example of a freely crawling form, we may ta.ke A iiiceba, 

 which crawls along by the aid of its pseudopodia. Conjugation has 

 recently been described in this form — two individuals meeting and 

 completely fusing as a preliminary to subsequent division. 



As an example of individuals which are from their position in 

 -easy reach of each other, we may instance Spivogyra, whose long 

 filaments lie side by side in the water. From these, short processes 

 arise, springing from the contiguous borders of neighbouring filaments ; 

 they pass towards each other, join, and their contents become united. 

 In some forms of Spiyogyva the processes differ from each other in 

 activity, the only indication of dimorphism present. 



Other examples of plants living in close contiguity and where 

 conjugation is remarkably easy, may be taken from the Confeyvoidea 

 isogam^. 



When we pass to the examination of the conditions under which 

 •dimorphism exists, we find that dimorphic forms are either (i) simple 

 cells, which are fixed and far apart from each other, or (2) individuals 

 that have undergone cell-differentiation, with the result that the 

 reproductive matter is contained within special reproductive cells. 

 In both cases, as we shall see very soon, there are certain difficulties 

 in the way of easy conjugation. 



As an example of simple fixed cells we may take the case of 



