344 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



the beginners of a new type within the species, that is, of a 

 modification of it. 



Such segregation can also occur in nature, and certainly often 

 does, by the chance isolation by geographic barriers of a few 

 individuals of a species, which are thus prevented from 

 breeding miscellaneously with the other members of the species, 

 but must breed together, thus tending to perpetuate their own 

 variations and idiosyncrasies. Such geographical isolation 

 has certainly led to the origination of many species modifica- 

 tions called geographic races or varieties, and with time and 

 the cumulation of these modifications through many genera- 

 tions, to many actual new species. 



It is possible also that certain physiological causes of the 

 enforced mating together of certain few individuals, thus pro- 

 ducing a sort of physiological segregation or isolation, may lead 

 to species modification. Personal antipathies, separate times 

 of coming to maturity, etc., may act as such agents of physi- 

 ological segregation. 



The Proof of Evolution. Despite our statement at the begin- 

 ning of this chapter that the fact of evolution is so generally 

 accepted that there is no longer any discussion of its proofs, 

 it may be advisable to state here succinctly on just what basis 

 the general belief in evolution rests. This basis is that of the 

 observed facts in four general biological subjects, namely, 

 paleontology, comparative anatomy and physiology, embry- 

 ology and development (ontogeny), and plant and animal 

 geography. 



By a study of animal fossils from all the different fossil- 

 bearing strata of the earth's crust, it is evident that in the 

 earliest geographical epochs only very simple animals lived, 

 such as Protozoa, the simpler invertebrates, and, perhaps, a few 

 simple Chordates. With the passing of time there came into 

 existence more specialized or higher invertebrates, then the 

 simplest true vertebrates, as the fishes and amphibians, and 

 finally the higher kinds, as the reptiles, birds and mammals. 

 The paleontological record is, in other words, plainly a record 

 of organic evolution. 



A careful comparison of the structure of different animals 



