46 COMPARATIVE PHrSIOLOGY. 



Thus, certain beetles reserable bees and wasps, which latter are 

 protected by stings. It is believed that such groups of beetles 

 as these arose by a species of selection ; those escaping enemies 

 which chanced to resemble dreaded insects most, so that birds 

 which were accustomed to prey on beetles, yet feared bees, would 

 likewise avoid the mimicking forms. 



4. Rudimentary Organs. — Organs -which 'were once func- 

 tional in a more ancient form, but serve no use in the creatures 

 in which they are now found, have reached, it is thought, their 

 rudimentary condition through long periods of comparative 

 disuse, in many generations. Such are the rudimentary mus- 

 cles of the ears of man, or the undeveloped incisor teeth found 

 in the upper jaw of ruminants. 



5. Geographical Distribution.— It can not be said that ani- 

 mals and plants are always found in the localities where they 

 are best fitted to flourish. This has been well illustrated within 

 the lifetime of the present generation, for the animals intro- 

 duced into Australia have many of them so multiplied as to 

 displace the forms native to that country. But, if we assume 

 that migrations of animals and transmutations of species have 

 taken place, this difficulty is in great part removed. 



■6. Paleontologfy. — ^The rooks bear record to the former exist- 

 ence of a succession of related forms ; and, though all the in- 

 termediate links that probably existed' have not been found, 

 the apparent discrepancy can be explained by the nature of 

 the circumstances under which fossil forms are preserved ; and 

 the "imperfection of the geological record." 



It is only in the sedimentary rocks arising from mud that 

 fossils can be preserved, and those animals alone with ^^^d 

 parts are likely to leave a trace behind them ; while if these 

 sedimentary rocks with their inclosed fossils should, owing to 

 enormous pressure or heat be greatly changed (metamorphosed), 

 all trace of fossils must disappear — so that the earliest forms 

 of life, those that would most naturally, if preserved at all, be 

 found in the most ancient rocks, are wanting, in consequence 

 of the metamorphism which such formations have undergone. 

 Moreover, our knowledge of the animal remains in the earth's 

 crust is as yet very incomplete, though, the more it is explored, 

 the more the evidence gathers force in favor of organic evolu- 

 tion. But it must be remembered that those groups constitut- 

 ing species are in geological time intermediate links. 



7. Fossil and Existing Species.— If the animals and plants 



