36 DOCTRINE OF EVOLUTION 



so many ways that we have learned to associate as the 

 distinguishing characteristics of these animals, and to 

 label ^'cat.'' The animals which we might see in a 

 walk of several blocks may reasonably be regarded as 

 offspring of the same pair of ancestors of a few years 

 back, even though they are dissimilar. We all know that 

 the kittens of one and the same litter vary : no two of 

 them are ever exactly alike in color or disposition or 

 voice or size, nor is any one identical with either of its 

 parents, although it may be necessary to employ exact 

 means of measuring them in order to demonstrate their 

 variation. The fact of difference, then, is surely not 

 inconsistent with even the closest ties of blood, and we 

 do not need to go beyond the scope of daily observation 

 to find that this is true in nature wherever we look. 



Should we extend our observations so as to include 

 the cats of Boston and Philadelphia and San Francisco, 

 the animals would probably vary over a wider range, 

 but they would be so similar to New York cats in their 

 make-up that we would have no difficulty in regarding 

 them and all the others of the United States as the 

 descendants of a single pair of ancestors, perhaps 

 brought over in the '^Mayflower." But why does this 

 view seem justified? Because experience has taught 

 us that the living things which resemble each other 

 most closely are those which are most intimately bound 

 by ties of blood and common heritage. It is ^'natural" 

 for relatives to resemble one another more than per- 

 sons not related, and for brothers and sisters to be more 

 alike than cousins. Science does not refer to something 

 outside everyday observation when it states that the 

 possession by two animals of a great body of similar 



