90 Studies in the Theory of Descent. 



consists in the secondary form in which the 

 species appears in the former originating through 

 the direct action of external conditions, whilst in 

 the latter this form most probably originates 

 through the indirect action of such influences. 

 The first half of the foregoing proposition is alone 

 capable of provisional proof, but it is in the 

 highest degree probable that the latter half is also 

 correct. Naturally we cannot say to what extent 

 the direct action of external conditions plays also 

 a part in true heterogenesis, as there have been 

 as yet no experiments made on its origin. That 

 direct action, working to a certain extent co-ope- 

 ratively, plays only a secondary part, while the 

 chief cause of the change is to be found in adapta- 

 tion, no one can doubt who keeps in view, for 

 instance, the mode of propagation discovered by 

 Leuckart in Ascaris nigrovenosa. In this worm, 

 the one generation lives free in the water, and 

 the other generation inhabits the lungs of frogs, 

 the two generations differing from one another 

 in size of body and structure of internal organs 

 to an extent only possible with the true Nema- 

 toda. 



To prevent possible misunderstanding, let it 

 be finally noted even if superfluous that the 

 changes causing the diversity of the two genera- 

 tions in seasonal dimorphism and heterogenesis 

 are not of such a nature that the value of different 

 " specific characters " can be attached to them. 



