THE GROWTH OF GROUPS 17 



relation in this respect is not quantitative. Unobtrusive 

 characters cannot become obtrusive by accumulation. 

 The pattern on a scale measuring less than a millimetre 

 in diameter cannot be rendered visible by accumulation. 

 Unobtrusive characters are not usually chosen as the 

 marks of identification of species. Consequently they 

 play no part in the problem of the origin of species. 

 But since they are definite and inheritable they are im- 

 portant in evolution, especially as we see it among mankind 

 at the present day. From this point of view there is a 

 difference between evolution and the origin of species. 

 The factors in evolution are unobtrusive characters, the 

 factors in the origin of species are obtrusive characters. 

 We are obliged to divide the problem in this way since 

 the conception of species has been forced upon us. There 

 is no essential difference between an obtrusive and an 

 unobtrusive character. They obtrude variously. But 

 since it is only the former that are considered to be 

 specific we may divide the problem as suggested. 

 If we investigate unobtrusive characters with great 

 accuracy we can split up any so-called species into many 

 sub-groups. In respect to its unobtrusive characters, 

 every group of vertebrate animals is polymorphic. In 

 taxonomy there is a progressive tendency to take into 

 account the less obtrusive characters and to consider 

 them as specific. The result is that the number of 

 species is becoming greater and greater, although the 

 species are becoming less obtrusive, so to speak, than 

 formerly. 



4. There is a real quantitative relation between 

 certain characters. Many characters which appear to 

 be units have arisen by accumulation, but the fact that 



c 



