WHAT IS THE ORIGIN OF SPECIES? 1 9$ 



some slight value in favor of the possessor, or against him, in 

 the struggle : the result would be the extinction of those less 

 well adapted and the preservation of the more favored — i.e., 

 a survival of the fittest. This is the law of natural selection. 



8. Darwin further added the principle of sexual selection ; 

 that is, that variations in habit, or even in color, are adapted 

 to cause a selection in pairing, which will lead to a further 

 perpetuation of certain characters and the isolation of varie- 

 ties into breeds, and thus the formation of species proper, or 

 larger groups of individuals, repeating by reproduction the 

 originally varietal characters of the few. 



9. Darwin noticed that divergence of characters is pro- 

 duced in animals and plants under domestication, gradually 

 and as the result of continued artificial selection ; hence he 

 inferred that the selection acting in nature will also tend to 

 perpetuate more and more markedly the strongly contrasted 

 varieties, the intermediate ones blending with the stronger 

 types ; thus, he believed, the differences, or gaps marking 

 species from species, are formed. 



There were other laws of variation which he noticed. 

 That use tends to develop, disuse to suppress characters, had 

 already been emphasized by Lamarck. Habit or custom 

 favors certain characters. Correlation of parts in growth 

 tends to cause variation in other parts, as adjustments to 

 changed organic conditions, and many others ; and the facts 

 of distribution of organisms were found in line with this theory 

 of origin of species, and paleontological succession is in har- 

 mony with it. In his sixth revised edition of " Origin of 

 Species," published in 188S, Darwin says definitely: "I be- 

 lieve that animals are descended from at most only four or 

 five progenitors, and plants from an equal or lesser number. 

 Analogy would lead me one step further, namely, to the be- 

 lief that all animals and plants are descended from some one 

 prototype."* And in the closing passage of the book he 

 sums up the essential points of his idea of the origin of 

 species, speaking of the laws by which all animals and plants 

 have been produced, thus: "These laws, taken in the largest 



* Vol. n. p. 299. 



