252 THE GENESIS OF SPECIES. [CHAP. 



This probability will be increased if the observations of 

 Dr. Bastian are confirmed by future investigation. Ac- 

 cording to his report, when the requisite conditions were 

 supplied, the transformations \vhich appeared to take place 

 (from very low to higher organisms) were sudden, definite, 

 and complete. 



Therefore, if this is so, there must probably exist in 

 higher forms a similar tendency to such change. That 

 tendency may indeed be long suppressed, and ultimately 

 modified by the action of heredity an action which would 

 increase in force with the increase in the perfection and 

 complexity of the organism affected. Still we might expect 

 that such changes as do take place would be also sudden, 

 definite, and complete. 



Moreover, as the same causes produce the same effects, 

 several individual parent-forms must often have been simi- 

 larly and simultaneously affected. That they should be so 

 affected at least that several similarly-modified individuals 

 should simultaneously arise has been seen to be a generally 

 necessary circumstance for the permanent duration of such 

 new modifications. 



It is also conceivable that such new forms may be en- 

 dowed with excessive constitutional strength and viability, 

 and with generative prepotency, as was the case with the 

 black-shouldered peacock in Sir J. Trevelyan's flock. This 

 flock was entirely composed of the common kind, and yet the 

 new form rapidly developed itself, " to the extinction of the 

 previously-existing breed" : 



Indeed, the notion accepted by both Mr. Darwin and 

 Mr. Herbert Spencer, and which is plainly the fact (namely, 

 that changes of conditions and incident forces, within limits, 

 augment the viability and fertility of individuals), harmon- 

 izes well with the suggested possibility as to an augmented 

 viability and prepotency in new organic forms evolved by 

 14 "Animals and Plants under Domestication," vol. i. p. 291. 



