LECTUEE XXXIY 

 ORIGIN OF THE SPECIFIC TYPE {continued) 



Illustration of phyletic evolution by an analogy — Reconciliation of Nageli and 

 Darwin — Unity of the specific type furthered bj' climatic variation — By natural 

 selection : illustration from aquatic animals — Direct path of evolution — Natural selec- 

 tion works in association with amphigony — Influence of isolation in defining the 

 specific type — Duration of the periods of constancy — The Siberian pine-jays — Species 

 are, so to speak, 'variable crystals' — Gradual increase of constancy and decrease of 

 reversions — Physiological segregation of species through mutual sterilitj^ — Romanes's 

 physiological selection — Breeds of domestic animals mutually fertile, presumably 

 therefore 'amiktic' species also — Mutual fertility in plant species — Mutual sterility 

 certainly not a condition of the splitting ujd of species— SjDlitting up of species without 

 amphigony — Lichens — Splitting up of species apart from isolation and mutual sterility, 

 Lepus variabilis. 



OuPt train of thought in the last lecture brought us back again 

 to the so-called ' indifferent ' characters, wliose occurrence is so often 

 used as evidence against natural selection, as a proof that evolution 

 is guided essentially by internal forces alone. But this objection was 

 based on a fallacy, for the fact that the first small variations are due 

 to internal processes in the germ-plasm does not imply that the whole 

 further course of evolution is determined by these alone, any more 

 than the fact that a sleigh requires a push to start it on its descent 

 down an inclined plane would imply that its rapid descent is due only 

 to the force of the push, and not at the same time to the attraction 

 of gravity. But the analogy is not quite sound, for the processes 

 within the germ-plasm which condition and direct variation do not 

 merely give them the first shove off: they are associated with every 

 onward step in the evolutionary path of the species, they impel it 

 further and further, and without tliese continual impulses the pro- 

 gressive movement would cease altogether. We have seen that, for 

 internal reasons, germinal selection continues to impel the varying 

 determinants further along the path on which they have started, and 

 thus gives them cumulative strength, and that it is in this way that 

 adaptations to the conditions of life are brought al)Out. The evolution 

 of the character of a species may thus be compared to the course of 

 a sleigli upon a level snow-surface which it could traverse in any 

 direction, but it is moved only by the inq)ulses received through 

 germinal selection. The conditions of life to whicli the varying parts 



