NO. I DISTRIBUTION OF THE ONYCHOPHORA CLARK II 



Iii the case of the onychophores the assumption that the Malayan 

 region is the center of distribution is somewhat arbitrary, though the 

 correctness of this supposition is strongly indicated by the fact that 

 the phylogenetic lines converge there. Under the very nearly uni- 

 form conditions which prevailed in the distant past there was no such 

 thing as a center of distribution ; new forms arose anywhere, and 

 immediately spread everywhere ; but as the surface of the earth be- 

 came differentiated into warm and cold regions and the mountain 

 ranges attained progressively to greater and greater heights, it 

 happened that, speaking broadly, the Malayan region as a whole re- 

 mained the region of least diurnal and seasonal variability, and of the 

 most delicately graded temperature differences, and therefore, as the 

 region of the most nearly permanent conditions and of the most 

 gradual differentiation in its physical and economic features, the 

 region of maximum physical and economic radius, and of least inter- 

 rupted progressive phylogenetical advance. 



Among the other groups of terrestrial organisms there are few, 

 if any, for which the Malayan region represents the sole center of 

 distribution as it may almost be said to do in the case of the ony- 

 chophores. Though in most cases, broadly speaking, the Malayan 

 region may reasonably be regarded as the chief, and possibly ultimate, 

 center of distribution, there are commonly additional centers of dis- 

 tribution each of which partakes more or less of the character of the 

 primary Malayan center. 



As has already been explained, it is characteristic of types which 

 have newly entered upon very favorable territory to vary very greatly, 

 and eventually to give rise to a large number of local forms, which, if 

 not subjected to the competition of more efficient intruders, may be 

 supposed, under fixed conditions, to persist for a very considerable 

 length of time, and which will be diversified in direct proportion to 

 the breadth of the physical and economic radius of the area. Such 

 specific abundance therefore indicates not the center of distribution 

 for any given type, but the periphery. Thus the great number of 

 species in the genus Oroperipatus occurring west of the crest of the 

 Andes indicates that this region, a region of small physical and 

 economic radius, represents the extreme western limit, and the maxi- 

 mum distance from the generative center, of the area inhabited by the 

 Peripatidse, while similar conditions in the genus Peripatus indicate 

 that their territory is only slightly less far removed from that center. 



The explanation of the distribution of the species of the family 

 Peripatidse, viewed in the light of what we know in regard to the dis- 

 tribution of other animal types, appears to be as follows : 



