HYPOTHESES OF VITAL DEVELOPMENT. 303 



during the embryonic state of each successive generation.* 

 Others, again, combine with these views the doctrine of 

 natural selection, by which, during the incessant changes 

 of geographical conditions, those varieties of plants and 

 animals best fitted for the new conditions will survive and 

 multiply, while those less adapted will gradually die out 

 and disappear. By such a process, these theorists contend, 

 such variations may go on till they amount to specific dis- 

 tinctions, and species themselves be converted into other 

 and higher genera. Perceiving that the ascent of life in 

 time that is, the ascent of life as shown by Geology co- 

 incides in a wonderful manner with the ascent from lower 

 to higher in living plants and animals, these hypothesists 

 admit the existence of a great creational plan, but seek to 

 explain its development through the operation of secondary 

 causes. Seeing that the physical phenomena of nature are 

 brought about by the operation of secondary causes, and 

 seeing that life is inseparably bound up with and de- 

 pendent on physical conditions, they seek to apply to the 

 one the same methods of research and reasoning which 



* A little reflection will enable the non-scientific reader to perceive the 

 force of these arguments. The eyes of animals inhabiting dark caverns 

 gradually degenerate, and in course of generations become merely rudi- 

 mentary. The feet of ruminants habitually frequenting arid plains gra- 

 dually lose the digits that add to the resistance of the hoof on soft and 

 swampy ground. Wingless birds, like the apteryx and ostrich, while 

 they have lost the power of flight, apparently through disuse, acquire by 

 degrees greater size and strength of limbs. Certain structures which are 

 transitory and rudimental in existing species, are persistent and developed 

 in extinct. Thus, the heterocercal or unequally-lobed tail, universal in 

 palaeozoic fishes, is still found in the embryo state of existing fishes, which 

 have chiefly homocercal or equally-lobed tails. The tapering caudal ver- 

 tebrae which appear in the embryo or chick of modern birds, was persis- 

 tent and characteristic in mesozoic species like the Archceopteryx. These 

 and many similar facts well known to anatomists give foundation and re- 

 liability to these theories, and evidently point the way to the Law which 

 has regulated and still continues to regulate the development of organic 

 existences. 



