122 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



"The terra 'species' was thus defined by the celebrated botanist 

 De Candolle: 'A species is a collection of all the individuals which 

 resemble each other more than they resemble anything else, which can 

 by mutual fecundation produce fertile individuals, and which repro- 

 duce themselves by generation, in such a manner that we may from 

 analogy suppose them all to have sprung from one single individual.' 

 And the zoologist Swainson gives a somewhat similar definition: 'A 

 species, in the usual acceptation of the term, is an animal which, in 

 a state of nature, is distinguished by certain peculiarities of form, size, 

 colour, or other circumstances, from another animal. It propagates, 

 after its kind, individuals perfectly resembling the parent; its pecu- 

 liarities, therefore, are permanent.' " l 



[As will have become apparent, the significant assumption 

 underlying classification is that the closest fundamental similarities 

 between animals (or plants) are found in the forms most closely 

 related and that the greatest differences are found in those forms which 

 are unrelated or at best very distantly related. The assumption 

 implies the idea of descent with modification, which is no more nor 

 less than evolution. Using this evolutionary basis, we can arrive at 

 an extremely satisfactory classification both of living and of extinct 

 forms; and there is no other basis of classification that works. 



The question might well be asked whether it is possible to test the 

 validity of the assumption that degrees of resemblance vary directly 

 with closeness of blood relationship ? Two direct tests of this may 

 be and have been made. The closest of blood relatives possible are 

 individuals that have been derived by the dividing of a single egg. 

 Armadillo 2 quadruplets have been shown to be thus derived, and 

 detailed studies of the closeness of resemblance existing between 

 members of a given set indicate that they are vastly more alike than 

 are the simultaneously born offspring of animals which give birth to 

 several young, but in which each young is derived from a separate egg. 

 If we use the index of correlation to indicate the degree of similarity 

 between individuals we find that ordinary brothers or sisters are only 

 about 50 per cent alike, while armadillo quadruplets are over 90 per 

 cent alike. Identical or duplicate twins in human beings are believed 

 to have an origin from one egg, after the fashion of the armadillo, 



1 From A. R. Wallace, Darwinism. 



* See H. H. Newman, The Biology of Twins (1917), University of Chicago 

 Press. 



