344 GENERAL ZOOLOGY 



distribution when compared with those of ancient lineage. 

 Such facts lead to the conclusion that specific types ori- 

 ginated in certain parts of the earth (centers of origin) and 

 slowly spread to other favorable localities. Furthermore, 

 it is well known that particular animals which have been 

 isolated (on islands, in valleys surrounded by high moun- 

 tain ranges, etc.) develop into types which differ in certain 

 characteristics from those that are free to wander. For 

 example, Australia has long been separated from other 

 continents and has been subjected to comparatively slight 

 climatic changes; its animals are primitive and show few 

 close affinities with those of other continents. The fauna 

 of the Galapagos Islands, while consisting of recent types, 

 is composed largely of unique and peculiar species. 



Generalizing from the facts of palaeontology and the 

 geographic distribution of animals, it may be said that 

 generalized types preceded and gave rise to those of special- 

 ized structure, that particular species arose in certain 

 localities and gradually spread to others, that species 

 isolated and thus prevented from mixing with others are 

 usually peculiar and rather closely adapted to local 

 conditions. 



Ontogeny. The "Law of Biogenesis" asserts that 

 "ontogeny repeats phylogeny" (page 60), and in general 

 the evidence for evolution from the development of indi- 

 vidual animals conforms with the sequence shown by 

 stratified fossil records. All vertebrates, for example, pass 

 through a fish-like stage (Fig. 115); palaeontology shows 

 that fishes were the first vertebrates to appear on the earth. 

 From such facts the older evolutionists reasoned that at 

 one time there were only fish-like vertebrates, and that 

 these later gave rise to other more specialized types. This 

 view has now been modified somewhat and at the present 

 time the same facts are generally interpreted as meaning 

 that all vertebrates are related because they pass through 

 similar stages in development. The "fish-like" stage in 

 the development of a mammalian embryo has gill slits and 



