12 PROBLEMS OF GENETICS 



biological research, I can scarcely suppose that it aids greatly 

 in the advances of other branches of our science. 



But why is it that, with these exceptions, the consequences 

 of the admittedly general acceptance of a theory of evolution 

 are so little reflected in the systematic treatment of living things? 

 Surely the reason is that though the systematist may be con- 

 vinced of the general truth of the evolution theory at large, he 

 is still of opinion that species are really distinct things. For 

 him there are still 'good' species and 'bad' species and his ex- 

 perience tells him that the distinction between the two is not 

 simply a question of degree or a matter of opinion. 



To some it may seem that this is mere perversity, a refusal 

 to see obvious truth, a manifestation of the spirit of the collector 

 rather than of the naturalist. But while recognising that from 

 a magnification of the conception of species the systematists 

 are occasionally led into absurdity I do not think the grounds 

 for their belief have in recent times been examined with the 

 consideration they deserve. The phenomenon of specific 

 diversity is manifested to a similar degree by living things be- 

 longing to all the great groups, from the highest to the lowest, 

 Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae, 

 and Bacteria, all present diversities of such a kind that among 

 them the existence of specific differences can on the whole be 

 recognised with a similar degree of success and with very similar 

 limitations. In all these groups there are many species quite 

 definite and unmistakable, and others practically indefinite. 

 The universal presence of specificity, as we may call it, simi- 

 larly limited and characterised, is one of their most remarkable 

 features. Not only is this specificity thus universally present 

 among the different forms of life, but it manifests itself in respect 

 of the most diverse characteristics which living things display. 

 Species may thus be distinguished by peculiarities of form, of 

 number, of geometrical arrangement, of chemical constitution 

 and properties, of sexual differentiation, of development, and of 

 many other properties. In any one or in several of these features 

 together, species may be found distinguished from other species. 

 It is also to be observed that the definiteness of these distinctions 



