EVOLUTION— PYCRAFT 227 



evolution of new characters. Living things, it is indeed true, have 

 "les defauts de leurs qualites." 



While we may look for, and find, similar results arising from like 

 stimuli they will always present contrasts because, as was pointed out 

 earlier in this address, no two organs, even of the same species, are 

 ever exactly alike in the composition of the substances which form 

 their tissues. And these differences become more marked as we pass 

 from individuals to species, and from species to genera, and so on, 

 in ever widening circles. But such organs show a striking con- 

 vergence as their functions tend to be changed by the rhythm of the 

 stimuli following changes in habit, or habitat, in the individual, in 

 the pursuit of its food, perhaps the most common inciting cause to 

 change. And thus it has come about that by this "convergent evolu- 

 tion" animals not even remotely related have come to assume a con- 

 spicuous likeness in their outward appearance. The Cetacea and 

 the extinct Ichthyosauria afford a striking illustration of this. 



But first let me say something of the Cetacean flipper. This, when 

 the whole group comes to be surveyed, presents some very remark- 

 able differences of which there is no indication until dissection is 

 resorted to. 



It will suffice for my purpose now, to bring to your notice some of 

 the more striking of these differences, though in all these cases the 

 changes in the skeleton, as compared with land dwellers, are pro- 

 found. No indication of the normal segments of the limb or of dig- 

 its are visible externally. Dissection shows that in the ziphoids and 

 in Platanista the structural change in the skeleton is less marked than 

 in any other cetacean, since the carpal region presents no more than 

 the early stages of decadence. 



In the rorquals we find the same elongated form of the flipper as in 

 the ziphoids, but here we have what must be interpreted as a very 

 striking and interesting case of "arrested development", for the 

 wrist bones fail to attain to complete ossification ; either no more than 

 a nodule of bone is found within the several cartilaginous forerun- 

 ners of the carpal bone, or these remain permanently cartilaginous, 

 even, in some cases, fusing one with another. Moreover, these bones 

 have become reduced in iiumber in all the Cetacea. In the killer 

 whale (Orcinus), and in the humpback (Megapfera), the vagaries of 

 development attain their maximum (pi. 4). In the first named the 

 ossification of phalangeals, or "finger joints", has become reduced to 

 narrow bands, which, in the terminal series of the second digit, are 

 represented only by their preaxial moieties. These ossified portions 

 are separated by enormous masses of cartilage, while the second and 

 third digits are curved round as though they had grown within a 



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