178 THE DOCTRINE OF DESCENT. 



behind or by the side of one another; hence on the 

 division of labour. This Haeckel has designated 

 divergent adaptation. It gives rise to the remark- 

 able polymorphism, which appears especially in the 

 marvellous forms of the Hydra tuba; and, higher up, in 

 the segmentation of the classes of the Termites and 

 the Bee, &c. 



So far as modification coincides with adaptation, the 

 direct adaptations hitherto discussed may be opposed 

 by a series of indirect adaptations. Among these may 

 be comprised a series of phenomena of which the causes 

 do not fall within the life of the individual, but are to 

 be sought in influences by which the parents were 

 affected. It is obvious that we here come into contact 

 with the province of heredity in a manner well known 

 to breeders. Thus H. v. Nathusius, in his studies on 

 the formation of the pig's skuU,''^ says : — " From the 

 facts here collected, it is plain that the transmission, the 

 transfer of the form of head from the parents to the 

 offspring, does not unconditionally ensue. If the form 

 of skull, which we will briefly term the cultivated form, 

 be the product of nutrition and mode of life, hence of 

 external influence, — if it can be differently formed in the 

 same individual, and is therefore not constant, — in that 

 case the heredity of this form cannot be spoken of 

 without qualification. The form itself will not be trans- 

 mitted to the offspring, but only the tendency to the 

 form. This may be inferred from the circumstance 

 that from generation to generation, and to a certain 

 degree, the form increases in peculiarity. If we rear a 

 common with a thoroughbred pig, and if we allow 

 exactly the same influences of nutriment and keeping 



