102 GENE.RAL ZOOLOGY 



of animals is not a mere grouping of species which resemble 

 each other to a greater or less extent, but is rather an attempt 

 at a statement of the truth of evolution, aiming to express 

 the relationships existing between animals, as shown not only 

 in their present structure but also by their development from 

 egg to maturity, and their history through geological time. 



While every species of animal tends to produce young 

 resembling itself, no two individuals of any species are ever 

 precisely alike. That the young of each species tend to 

 resemble their parents, we say is due to heredity ; that they 

 never exactly resemble them, we say is due to variation. Of 

 the causes underlying heredity and variation we know almost 

 nothing. We notice that in the course of time various forms 

 of life have appeared on the earth, and that on the whole 

 there has been a gradually increasing complexity in animal 

 structure through geological time, the more complex animals 

 following the simpler, as the environment has changed and 

 new conditions have arisen. Because of the incompleteness 

 of the knowledge of the evolution of life on the earth, we 

 can do little more than call attention to some of the second- 

 ary factors in evolution, without attempting to measure, ex- 

 cept in a rough way, their relative importance. 



Natural Selection. With the principle of natural selection 

 are associated the names of Charles Darwin and Alfred Russel 

 Wallace, who independently stated it in papers published in 

 1858 (see p. 450). The principle had been recognized before, 

 but as stated by Darwin it was reenforced by such a wealth 

 of illustration that it compelled the attention of the scien- 

 tific world. 



In introducing the discussion of natural selection, Darwin 

 makes use of the principle of artificial selection among domes- 

 ticated animals, that is, the selection by man of certain 

 individuals having peculiarities which he wishes to preserve. 

 These individuals are allowed to breed together, and, in the 



