74 Resemblances and Di'Qerences Among hiving Things 



(2) There are the facts of heredity; individuals re- 

 semble one another according to the degree of relationship. 



(3) There are the facts of variation; not only do in- 

 dividuals of the same ancestry differ among each other, but 

 offspring differ from their immediate parents. 



(4) Diverging individuals become the ancestors of di- 

 verging strains, breeds, varieties, or eventually species. 



Evolution Familiar to Everybody 



We may be able to imagine the opposite of each of these 

 sets of facts. But the alternatives do not fit our actual ex- 

 perience. We can imagine living things originating anew 

 out of non-living objects in as whimsical an order as we 

 please — or out of nothing at all. We can imagine plants 

 and animals, old kinds or new kinds, " created " right on the 

 spot with no living thing to precede each new individual. 

 But this picture is contrary to all of our positive knowledge 

 about plants and animals. 



We can imagine living things without the facts of 

 heredity. While acorns arise from oaks and give rise under 

 suitable conditions to new oaks, we can imagine oak trees 

 giving rise to opossums, or any kind of living thing giving 

 rise to almost any other kind without restriction. But that 

 is not the way the plants and animals which we know do 

 actually behave. 



Again, we can imagine all the individuals of a particu- 

 lar kind being exactly alike — not only offspring like parents, 

 but all members of a species exactly alike, and individuals 

 quite indistinguishable from one another, except perhaps for 

 age or sex. Indeed, in many species of plants and animals 

 this is so much the case that it is almost impossible to dis- 

 tinguish one individual from another, at least by casual in- 

 spection. Among the very simplest plants and animals, 

 and among certain kinds of insects, the variation from indi- 

 vidual to individual is very slight. Yet our common experi- 

 ence is such that, along with the facts of resemblance in 



