402 GENERAL PRINCIPLES 



fail to develop at all. In adult Amphibia and Reptiles a pair of 

 these vessels persists to form the aortic arches. 



814. The conclusion seems evident. The gill arches and 

 their blood vessels are a fish character, and their presence in the 

 terrestrial vertebrates can only mean that as vestigeal organs 

 they hark back to fish-like ancestors. But this is only one of 

 many anatomical puzzles which can be explained in this way. 



815. The remarkable parallelism which appears between 

 the Taxonomic series, the Phylogenetic series and the On- 

 togenetic series assuredly warrants the formation of a provis- 

 ional hypothesis of the origin of species by descent. The 

 value of any hypothesis is gauged by the extent to which it 

 explains phenomena and by the help it gives in the discovery 

 of new facts. We shall proceed to apply this test, but let 

 us first enquire what causes might be supposed to bring 

 about a change in species. 



8 1 6. The Struggle for Existence. Taking the whole world 

 into account and year after year, there is on the average no 

 great change in the number of individual organisms. Locally 

 and for brief periods there frequently occurs an increase or a 

 decrease in the number of a given species. But extended 

 changes of this kind are comparatively rare even for a single 

 species. This indicates that each pair of adult individuals 

 at the time of death have provided a progeny of the same num- 

 ber to fill the gap. But the rate at which even the slowest 

 breeding animals reproduce is much in excess of this, and for 

 many the rate is many thousand-fold greater. Many plants 

 produce several thousand to several million seeds in a season 

 and, in the case of perennials, this is done for many years. 

 For many animals a single brood of eggs ranges from many 

 thousands to many millions. The conger eel may produce five 

 or six million eggs, while the female Ascaris is credited with a 

 brood of sixty-four million eggs. Yet in these cases only one 

 or two eggs can ultimately have developed into an individual 



