v.] IN THE THEORY OF NATURAL SELECTION. 295 



first arose ; for they derived this form of reproduction from 

 their unicellular ancestors. 



We know that organs and characters which have persisted 

 through a long series of generations are transmitted with 

 extreme tenacity, even when they have ceased to be of any 

 direct use to their immediate possessors. The rudimentary 

 organs in various animals, and not least in man, afford very 

 strong proofs of the soundness of this conclusion. Another 

 example has only recently been discovered in the sixth finger, 

 which has been shown to exist in the human embryo ^ a part 

 which has only been present in a rudimentary form ever since 

 the origin of the Amphibia^. Superfluous organs become 

 rudimentary very slowly, and enormous periods must elapse 

 before they completely disappear, while the older a character 

 is, the more firmly it becomes rooted in the organism. What 

 I have above called the physical constitution of a species is 

 based upon these facts, and upon them depend the tout ensemble 

 of inherited characters, which are adapted to one another and 

 woven together into a harmonious whole. It is this specific 

 nature of an organism which causes it to respond to external 

 influences in a manner different from that followed by any 

 other organism, which prevents it from changing in any way 

 except along certain definite lines of variation, although these 

 may be very numerous. Furthermore these facts ensure that 

 characters cannot be taken at random from the constitution of 

 a species and others substituted for them. Such a variation 

 as a mammal wanting the firm axis of the backbone is an im- 

 possibility, not only because the backbone is necessary as a 

 support to the body, but chiefly because this structure has been 

 inherited from times immemorial, and has become so impressed 

 upon the mammalian organization that any variation so great as 

 to threaten its very existence cannot now take place. The 

 view here set forth of the origin of hereditary variability by 

 amphigonic reproduction, makes it clear that an organism is in 



^ Compare (i) Bardeleben, ' Zur Entwicklung der Fusswurzel,' Sit- 

 zungsber. d. Jen. Gesellschaft, Jahrg. 1885, Feb. 6; also ' Verhandl. d. 

 Naturforscherversammlung zu Strassburg,' 1885, p. 203 ; (2) G. Baur, 

 'Zur Morphologic des Carpus und Tarsus der Wirbelthiere,' Zool. 

 Anzeiger, 1885, pp. 326, 486. 



■^ In frogs the sixth toe exists in the hind legs as a rudimentary pre- 

 hallux. Compare Born, Morpholog. Jahrbuch, Bd. I, 1876. 



