v.] IN THE THEORY OF NATURAL SELECTION. 307 



of species, genera, orders, etc. 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 complexity 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 

 idioplasm. 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 

 consequence 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 formations. 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 inca- 

 pable of transformation, and might, under favourable conditions, 

 continue to exist for an indefinite period. But this reply would 

 introduce a new hypothesis in direct antagonism to Nageli's 

 theory, which assumes that the variability of idioplasm takes 

 place as the consequence of mere growth, and necessarily 

 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 genera- 

 tions were indefinitely prolonged'. Hence there can be no 



1 1. c, p. 118. 

 X 2 



