196 NATURAL SCIENCE. Sept., 



Vorticella. These are fixed by a stalk to weeds or stones, and of 

 course they are unable to cluster close to each other for lack of food. 

 In order that conjugation may occur, the Vorticella generally divides 

 into two halves, one of which re-divides into from two to eight parts. 

 These become ciliated and free swimming; they swim to and conjugate 

 with an ordinary stalked Vorticella. 



By far the larger number of instances of dimorphism are to be 

 found in higher and more complex animal and vegetable forms. 

 Here, the individual consists of cells, some of which exist for protec- 

 tion, others for movement, others for purposes of nutrition, and 

 others, again, are the reproductive cells. Here, we meet with two 

 distinct kinds of dimorphism, both of which may be termed sexual. 

 In the first place, the special reproductive cells, or gametes, are 

 different from each other, and this we may term dimorphism of the 

 gametes; and in the second place, the whole individual may be 

 dimorphic, dimorphism of the individual. 



Thus, in Hydra, one individual specimen produces two kinds of 

 reproductive cells or gametes, and these may be termed male and 

 female gametes. One individual hydra does not, however, differ 

 from another individual hydra — there are no male and no female 

 hydras : the individual may be termed hermaphrodite, from its pro- 

 ducing both male and female gametes. In by far the larger number 

 of plants and animals a division of labour occurs, and the male 

 gametes are produced by one individual, the females by another, so 

 that in this case we may speak of a male and female individual, more 

 especially as these individuals generally come to differ from each 

 other in qualities other than that of merely producing different 

 reproductive cells. 



Dimorphism of the Gametes. — Now it is very probable that 

 the first indication of dimorphism was the production by individuals 

 not dimorphic themselves of gametes which were dimorphic, and that 

 dimorphism of the individual was a subsequent product. The reason 

 for this belief is the fact that the more complex double dimorphism 

 prevails to a greater extent in higher than in lower forms of life. 

 Concentrating our attention upon the dimorphism of the gametes, we 

 find that, as already indicated, it is associated with certain difficulties 

 in the way of conjugation. In Vorticella these are obvious, and 

 hardly less so among animals which have undergone cell-differentiation. 

 Even where individual dimorphism has furnished special ducts and 

 organs for bringing the gametes together, these, on account of their 

 small size (they are generally microscopic) have got to travel what is 

 for them a considerable distance before conjugation can occur. 

 While we may allow that dimorphism is present in association with 

 difficulties in the way of conjugation, we have only taken the first 

 step in our investigation ; we have yet to understand how dimorphism 

 can overcome these difficulties. 



We might suppose that the production of moving gametes would 



