IN THE THEORY OP NATURAL SELECTION. 299 



orders, &c. would be of approximately equal length respectively, at 

 least in forms of equal structural complexity. The time required 

 by the idioplasm to undergo such changes as would constitute 

 transformation into a new species ought to be always the same at 

 equal heights in the scale of organization, that is, with equal com- 

 plexity in the molecular structure of the idioplasm. It appears to 

 me to be a necessary consequence of Nageli's theory that the causes 

 of transformation lie solely in this molecular structure of the idio- 

 plasm. If nothing more than a certain amount of growth, and 

 consequently a certain period of time during which the organism 

 reproduces itself with a certain intensity, is required to produce a 

 change in the idioplasm, then we must conclude that the alteration 

 in the latter must take place when this certain amount of growth 

 has been reached, or after this certain period has elapsed. In other 

 words, the time during which a species exists from its origin as a 

 modification of some older species, until its own transformation into 

 a new one must be the same in species with the same degree of 

 organization. But the facts are very far from supporting this con- 

 sequence of Nageli's theory. The duration of species is excessively 

 variable : many arise and perish within the limits of a single 

 geological formation, while others may be restricted to a very small 

 part of a formation ; others again may last through several forma- 

 tions. It must be admitted that we cannot estimate the exact 

 position of extinct species in the scale of organization, and the 

 differences may therefore depend upon differences of organization : 

 or they may be explained by the supposition that certain species 

 may have become incapable of transformation, and might, under 

 favourable conditions, continue to exist for an indefinite period. 

 But this reply would introduce a new hypothesis in direct anta- 

 gonism to Nageli's theory, which assumes that the variability of 

 idioplasm takes place as the consequence of mere growth, and ne- 

 cessarily depends upon molecular structure. Nageli himself asserts 

 that the essential substance (idioplasm) of the descendants of the 

 earliest forms of life is in a state of perpetual change, which would 

 continue even if the series of successive generations were indefinitely 

 prolonged *. Hence there can be no rest in the process of change 

 which the idioplasm must undergo ; and this is as true of each 

 single species as it is of the organic world taken as a whole. We 



1 1. c., p. 1 1 8. 



