1887.] NATURAL SCIENCES OF PHILADELPHIA. 347 



fication in a determinate direction (not hap-hazard, through fortui- 

 tous variation,) would be conditioned by definite retroactions be- 

 tween the organism and its environment, and thus gradually stamp 

 it with peculiar and mechanically advantageous features. This 

 view should not be lost sight of; and, while it has been demonstra- 

 ted to the satisfaction of all anatomists, (except P. Albrecht, whose 

 views as to the primitive nature of the Cetacea are not borne out 

 upon either morphological or palseontological grounds,) that, the ab- 

 errant forms alluded to above, are in all probability descendants of 

 land and semi-amphibious types, we are forcibly reminded by these 

 examples, of how the evolution of strongly marked types is directly 

 conditioned by the peculiar nature of their specialized environment. 



This brings us to the question of distribution in time, or the se- 

 quence of forms. Here, palaeontology comes to the aid of the mor- 

 phologist and affords the final proof that the Fishes are veritably 

 the oldest of the limb-bearing vertebrates. The evidence derived 

 from ontogeny, comparative morphology, taxonomy and palaeon- 

 tology is, therefore, in accord and quite conclusive as to the main 

 fact of the primitiveness of the fish-like type, which is, therefore, the 

 only one which can be regarded as aflfording the direct ancestral 

 means through which the structural condition of the limbs of high- 

 er forms may be reached. 



The mediate or indirect ancestry of the higher vertebrate types, 

 was undoubtedly, one in which there obtained a condition even more 

 ' primitive than in the most generalized of the limb-bearing fishes. 

 Such a type was one which probably diflered from the most gener- 

 alized fish, in having the metameric elements of the paired limbs 

 distinct for each segment ; and not coalesced or fused into a more or 

 less closely connected longitudinal series, covered by a simple, com- 

 mon fold of integument as in the latter. The only types which ap- 

 proach such a condition, are the existing Errant Annelids or Worms. 

 These, or some peculiar generalized forms of Worms were therefore, 

 the probable prototypes of the vertebrates as maintained by Dohrn 

 and Semper, and, thanks to the brilliant researches of Bateson upon 

 Balanoglossus, such a view has more in its favor now than ever be- 

 fore. Yet, here again the unwary must be cautioned, since Balano- 

 glossus, living as it does buried in the sand, has probably undergone 

 some retrogressive metamorphosis, so that it has been itself modified 

 and diverted out of the path leading in the direction of the descent 

 of the true vertebrate forms. 



