SUCCESSION OF MAMMALIA. 443 



change of actions, when habitual, becomes another cause of 

 altered structure ; that the more frequent employment of cer- 

 tain parts or organs leads to a proportional increase of develop- 

 ment of such parts ; and that as the increased exercise of one 

 part is usually accompanied by a corresponding disuse of 

 another, this very disuse, by inducing a proportional degree 

 of atrophy, becomes an additional element in the progressive 

 mutation of organic forms. 



Another theorist calls to mind the instances of sudden 

 departure from the specific type, manifested by a malformed 

 or monstrous offspring, and quotes the instances in which such 

 malformations have lived and propagated the deviating struc- 

 ture. He notes also the extreme degrees of change and of 

 complexity of structure undergone by the germ and embryo 

 of a highly organized animal in its progress to maturity. He 

 speculates on the influence of premature birth, or on a some- 

 what prolonged foetation, in establishing the beginning of a 

 specific form different from that of the parent. 



Darwin and Wallace, to explain the origin of species, 

 combine the principle of "the contest for existence" with 

 those of " accidental variety," " inherited variety," and " the 

 influence of external circumstances and internal adaptability," 

 as co-efficients in altering specific characters. Each theorist 

 invokes the requisite duration of time. 



But observation of the actual change of any one species 

 into another, through any or all of the above hypothetical 

 transmuting influences, has not yet been recorded. And past 

 experience of the chance aims of human fancy, unchecked 

 and unguided by observed facts, shews how widely they have 

 ever glanced away from the gold centre of truth. 



Facts that oppose some of the surmises on the origin of 

 species have been elsewhere pointed out by the writer.* The 



* Description of the skull of the Troglodytes gorilla, Feb. 1848. Trans. 

 Zool. Soc, vol. iii., p. 414. 



