366 THE EVOLUTION THEORY 



and if the cell gets into a wrong position in the course of develop- 

 ment because of some disturbance or other, a wing may develop from 

 it in that position if the conditions are not too utterly divergent. 

 These heterotopic phenomena aiford a further proof of the existence 

 of determinants, because they are quite unintelligible without the 

 assumption of 'primary constituents' or Anlagen. 



The hypothesis of determinants in the germ-plasm is so funda- 

 mental to my theory of development that I should like to adduce 

 another case in its support and justification. The limbs of the 

 jointed-footed animals, or Arthropods, originally arose as a pair on 

 each segment of the body, and they were at first alike or very similar 

 both in their function and in their form. We find illustration of 

 this in the millipedes, and still more in the species of the interesting 

 genus Peripatus, which resembles them externally, as well as in the 

 swimming and creeping bristle-footed marine worms (ChaBtopods) 

 belonging to the Annelid phylum. We can quite well picture to 

 ourselves that the whole series of these appendages was represented 

 in the germ-plasm by a single determinant or group of determinants, 

 which only required to be multiplied in development. Without 

 disputing whether this has really been the case in the primitive 

 Arthropods or not, it is certain that it can no longer be the case 

 in the germ-plasm of the Arthropods of to-day. In these each pair 

 of appendages must be represented by a particular determinant. 

 We must infer this from the fact that the several pairs of these 

 appendages have varied transmissibly, independently of each other, 

 for some are jaws, others swimming legs, or merely bearers of the 

 gills or of the eggs; others are walking legs, digging legs, or jumping 

 legs. In Crustaceans a forceps-like claw is often borne by the first 

 of the otherwise similarly constructed appendages, or also by the 

 second or the third, or there may be no forceps, and so on ; in short, 

 we see that each individual pair has adapted itself independently 

 to the mode of life of its species. This could only have been possible 

 if each was represented in the germ-plasm by an element, whose 

 variations caused a variation only in that one pair of legs, and in 

 no other. 



It may perhaps be objected that the differences in the appendages 

 may quite well have had their origin simply during the development 

 of the animal, while the primary constituents were the same for all, so 

 that a single determinant in the germ-plasm would suffice. But this 

 could only be the case if the differences depended not on internal but 

 on external causes, that is, if the same primary constituents gave rise 

 to a set of appendages which became different because they were 



