252 THE GENESIS OF SPECIES. [Ciup. 



Tills probability will be incrcasccl if the observations of 

 Dr. I5astian are confirined by future investigation. Ao- 

 conling to his report, when the requisite conditions were 

 supplied, the transformations which appeared to take place 

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

 and comj)lete. 



Therefore, if this is so, there must proba])ly exist in 

 higher forms a similar tendency to sucli change. That 

 tendency may indeed be long suppressed, and ultimately 

 modified by the action of heredity — an action which w(juld 

 increase in force with the increase in the j)erfection 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 efTects, 

 several individual parent-forms must often have been simi- 

 larly and simultaneously affected. That they should be so 

 afTected — at least that several similarly-jnodified 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 

 previoitsly-exlsting breed?'' " 



Indeed, the notion accepted by both Mr. l^arwin 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 



** "Auimald and Plants under Domestication," vol. i. p. 291. 



