144 MENDELISM chap. 



nature. They serve as convenient labels, but are 

 not essential to the conception of species. A 

 systematist who defined the wild sweet pea could 

 hardly fail to include in his definition such characters 

 as the procumbent habit, the tendrils, the form of 

 the pollen, the shape of the flower, and its purple 

 colour. Yet all these and other characters have 

 been proved to depend upon the presence of definite 

 factors which can be removed by appropriate cross- 

 ing. By this means we can produce a small plant 

 a few inches in height with an erect habit of growth, 

 without tendrils, with round instead of oblong pollen, 

 and with colourless deformed flowers quite different 

 in appearance from those of the wild form. Such a 

 plant would breed perfectly true, and a botanist to 

 whom it was presented, if ignorant of its origin, 

 might easily relegate it to a different genus. Never- 

 theless, though so widely divergent in structure, 

 such a plant must yet be regarded as belonging to the 

 species Lathyrus odoratus. For it still remains fertile 

 with the many different varieties of sweet-pea. It 

 is not visible attributes that constitute the essential 

 difference between one species and another. The 

 essential difference, whatever it may be, is that 

 underlying the phenomenon of sterility. The visible 

 attributes are those made use of by the • systematist 

 in cataloguing the different forms of animal and 

 plant life, for he has no other choice. But it must 

 not be forgotten that they are often misleading. 

 Until they were bred together Euralia wahlbergi 

 and E. mima were regarded as perfectly valid species, 

 and there is little doubt that numbers of recognised 

 species will eventually fall to the ground in the same 



