76 Resemblances and Differences Among Living Things 



proportion to degrees of relationship, there are also differences 

 that make each individual practically unique. 



We can imagine the inhabitants of the earth to remain 

 practically unchanged from generation to generation, from 

 century to century. This is conceivable, whether individu- 

 als in a given species differ from each other much or little. 

 The facts of experience, however, show us that among the 

 offspring which differ from their parents are some that initi- 

 ate new lines, which remain consistently different from the 

 ancestral stock. 



Throughout centuries observers and philosophers have 

 felt convinced that new species appear from time to time 

 and must in past ages have descended from ancestors some- 

 what different from themselves. There has never been, until 

 our own times, a sufficient accumulation of accurate observa- 

 tions to tell us whether new species gradually depart from 

 the ancestral types, or whether a new form arises by means 

 of a rather sudden departure from the parental form. Much 

 of the speculation and uncertainty and confusion have come 

 from the lack of knowledge on this point; but whoever 

 knows the facts is satisfied that there has been descent with 

 modifications. 



Classification of living things is a highly artificial proc- 

 ess, since we cannot know species or classes as existing en- 

 tities. Yet it is by no means arbitrary. The resemblances 

 that lead us to group some forms together and the discrimi- 

 nations by which we separate nearly similar individuals into 

 subdivisions represent real facts, however we may account 

 for them. The persistent study of plant and animal forms 

 leads us to group them into arrangements that suggest pro- 

 gressive divergence from common ancestors in exactly the 

 same way as a family tree shows divergence from common 

 ancestors. Succession by descent with divergence constitutes 

 the essential facts of organic evolution and is thus supported 

 by the facts of resemblance and difference among living 

 things. 



