4 PROBLEMS OF EVOLUTION 



(3) More young are born in every species than would be 

 required to keep up its numbers, supposing that there were no 

 natural check at work to prevent increase. He instanced especi- 

 ally the elephant, the slowest of breeders. If allowed to multiply 

 unchecked, elephants would soon people the earth. 



(4) In the struggle for existence all favourable variations are 

 singled out for survival by Natural Selection, which acts in the 

 same way as artificial selection. And thus, as the breeder has 

 produced the many varieties of domestic pigeons, nature has pro- 

 duced all the many species of the animal and vegetable kingdoms. 

 Sexual selection supplements Natural Selection. 



Evidence (i) The science of embryology has shown that each individual 

 as ^ n one ^ t ^ ie h'gh er s P ec i es f animals goes through many of the 

 taken stages through which, according to the theory, the species has 

 pla e gone in the process of evolution. 



In scientific language this may be expressed thus : the onto- 

 geny, the development of the individual, as it proceeds, recapitu- 

 lates briefly the phylogeny, the history of the evolution of the 

 species. Thus man is at first a one-celled Protozoon, then an 

 agglomeration of undifFerentiated cells ; at a later stage he has 

 gill slits, like those of a shark, though no functioning gills. 



(2) Palaeontology shows an advance from the lower to the 

 higher forms. We find : 



(a) In the primary rocks fishes and amphibians, and in the 

 most recent of them some reptiles. 



(b) In the secondary rocks reptiles are dominant on land, in 

 the sea, and in the air. Pterodactyls, winged lizards, some 

 small and some large, are kings of the air. But birds are begin- 

 ning to appear. There is the archaeopteryx, a true bird with 

 many reptilian characteristics, found as a fossil in the Bavarian 

 lithographic stone, and the toothed birds of America belong to 

 this period. Small marsupials, allied to the existing kangaroo, 

 are the only mammals. 



(<:) In the tertiary, birds of a less reptilian type are found in 

 every quarter of the globe, but dominant everywhere are the 

 higher mammals, called from the manner of their growth as 

 embryos, Placental. In this period it may be considered as 



