MOMENTUM IN EVOLUTION. 270 



being thus removed along certain lines by the action of natural selection, 

 a definite direction may be given to the course of evolution which the 

 organism will continue to follow irrespective of natural selection? 



T shall probably be told that all organs vary, and that when any 

 particular organ has reached the optimum size natural selection will 

 prevent it from going further by eliminating the unfavourable varia- 

 tions, i.e., those which exhibit further increase. It may be admitted 

 that the organ in question will probably exhibit variation in size after 

 reaching the optimum, due to differences in nutrition and other pecu- 

 liarities of the individual environment ; but I fail to see how, in the 

 absence of the gland which produces the specific controlling secretion, 

 and which we have assumed to have been already eliminated, there are 

 likely to be any variations of a minus character suitable for natural 

 selection to work upon. In other words, it appears to me probable that 

 natural selection, having once let go her control of growth, would be 

 unable to regain it. In order that she might do so it would be neces- 

 sary either that the glandular organ which originally produced the 

 inhibiting hormone should be again developed or that some other organ 

 should take its place. It is, however, generally admitted that an organ 

 once lost is never redeveloped, and it does not seem likely that any 

 other glandular organ, which we may suppose to be already occupied 

 in producing a specific secretion for some other purpose, would be able 

 to take on new duties and provide the necessary control before it was 

 too late to save the organism from destruction. 



If there is a possibility of any cumulative effect from generation 

 to generation there seems no reason why, under these circumstances, 

 increase of size should not continue indefinitely until it becomes incom- 

 patible with existence. Have we any right to assume any such cumu- 

 lative effect? I think we have, for we know very well that the whole 

 ontogeny of any one of the higher animals is nothing but the accumu- 

 lation of a number of successive stages which have been added one after 

 the other in the individual lifetimes of past generations. This at any 

 rate is the teaching of the recapitulation hypothesis, in the truth of 

 which I, for one, am a convinced believer. We also know from the 

 facts' of embryology that as each successive stage is added there is a 

 tendency both towards an increase in the length of time occupied in 

 development and also towards compression and abbreviation of the 

 earlier stages so as to make room for new chapters of the record. 



It seems, therefore, not unreasonable to assume that any increment 

 in size which is gained by an individual animal or one of its organs 

 before the period of reproduction, or before the germ cells which will 

 give rise to the next generation are matured, and which is the result of 

 the removal of some controlling factor, will tend to be inherited in the 

 offspring in a cumulative fashion. If not, why hove other features in 

 the ancestral history been accumulated by heredity? It may be said 

 that after the maximum rate of growth has once been attained there will 

 be no further increase in the size of the organ, but I think there will, if 

 only because there will be a slightly increased time available, owing to 

 the lengthening of the period of development, in which growth may 



