122 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
“The term ‘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 zodlogist 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.’ ”’ ? 
{As will have become apparent, the fundamental 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? 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, 
tFrom A. R. Wallace, Darwinism. 
?See H. H. Newman, The Biology of Twins (1917), University of Chicago 
Press. 
