XXI.] THE TENDENCY TO UNISEXUALITY. 349 



There must be a general tendency toward dichog- 

 amy or unisexuality. All the higher animals are 

 male or female, and some of the plants are so, also. 

 The great majority of plants, however, are still her- 

 maphrodite. All our common fruits have what the 

 botanists call perfect flowers, that is, those which 

 contain both male and female elements. Yet nearly 

 all hermaphrodite plants develop their stamens and 

 pistils at different times, so that the flower can- 

 not fertilize itself. This, we suppose, is in conse- 

 quence of the fundamental law that the constructive 

 and destructive changes upon which the female and 

 male elements respectively rest — or anabolism and 

 katabolism — cannot proceed simultaneously. In many 

 plants, self-fertilization is prohibited or hindered by 

 this simplest of all methods, — the different or alter- 

 nate maturing of the sex members. But the plant 

 often goes further than this, and the pistil or seed- 

 bearing member refuses to accept the pollen from the 

 same flower, or even from any flower on the same 

 plant ; or, to transpose the statement, the pollen is 

 impotent upon its own sisterhood of pistils. It is 

 difficult to account for the physiological origin of this 

 im potency, although we should expect that pollen - 

 bearing members which are prevented from fertilizing 

 associated pistils might in time develop pollen which 

 would be incapable of fertilizing them ; but its use to 

 the species is obvious, inasmuch as it insures cross - 

 fertilization, and thereby tends to strengthen or revi- 

 talize the species. Darwin was among the first to 

 study this subject, and he published a list of plants 

 which are sterile with their own pollen ; but none of 

 the fruits are in his list. 



