CHAPTER XXIII 



CLASSIFICATION AND EVOLUTION 



The animals that we have examined in the foregoing 

 chapters have been chosen with a view to 

 SJf e 8 c 8 |e f 8l !atlon ! their serving, among other things, as examples 

 of the principal kinds of creatures that con- 

 stitute what is known as the Animal Kingdom. Ex- 

 plicitly or implicitly, the study of different objects of 

 any kind must always proceed by a recognition of their 

 resemblances and differences, but the number of 

 different kinds of animals is so enormous that it is quite 

 impossible to study them without arranging them in an 

 orderly classification according to their degrees of likeness. 

 We have seen that no two individual animals are wholly 

 alike. The offspring of any parent are always unlike it 

 and unlike one another. Even so-called "identical twins," 

 whatever may be the case at their birth, become to some 

 degree different by the different action of their surroundings 

 upon them as they grow up. Heredity, in fact, does not pro- 

 duce absolute resemblance, but is qualified by what, using the 

 term in its widest sense, we may call variation, whether it be 

 due to an unlikeness in the offspring at birth or acquired by 

 the impress of the surroundings during their lifetime. At the 

 same time, the likeness between the offspring of any parent 

 is, on the average, greater than their likeness to individuals 

 descended from other parents, and in this fact we find 

 the first degree of resemblance between animals. For 

 practical purposes, however, the resemblance between 

 members of a family (in the ordinary sense of the word) 

 is useless in classification, on account of the vast number 

 of small. divisions it gives and the impossibility of identi- 



