[108] THE EVOLUTION OF THE FINS OF FISHES. 1083 



XYII. — A STATEMENT OF SOME OBJECTIONS AND CONCLUDING^ RE- 

 MARKS. 



I am aware that it may be objected that the development of au oi)i.s- 

 thureln Chinucra 7nonstrosa,vfli\lQ it is abseutin C. phimhea, would seem to 

 militate a good deal against the hypothesis here set forth, since it might 

 be asked why the same forces should not produce the same effects in 

 both these forms otherwise so nearly allied. Nor is this the only objec- 

 tion which mightfbe raised, for why is it, it may be imiuired, that cer- 

 tain physostomous forms, asAlosa for examjjle (which is evidently lower 

 in respect to the development of the air-bladder than the Fhi/socJistl), 

 should exhibit a specialization of caudal development not found to oc- 

 cur in those forms which are evidently more advanced when their entire 

 organizations are considered in this comparison *? But none of these or 

 the numerous kindred objections which might be raised need dismay us 

 in our inquiry for the reason for the present morphological composition 

 of the caudal fins of fishes, in which we have been at least measurably 

 successful in demonstrating something akin to order in the elucidation 

 of the most important of all the questions relating thereto, namely, that 

 the skeletal parts are, beyond any doubt whatever, serially homologous. 

 When once this fact is appreciated, and it is at the same time understood 

 that the skeletogenous layers are also homologous throughout the dif- 

 ferent groups of fishes, it will be evident to any reasonable person that 

 the causes which would lead to the production of very different mor- 

 phological eft'ects would need but little modification to make them ef- 

 fective. This is all the more evident when it is remembered that the 

 habits of the different groups vary very greatly, and that the si)tcial 

 modification of some part through the exercise of some special habit 

 must affect other parts correlatively, so that it becomes impossible to 

 predict what the effect will be upon the many parts taken singly in such 

 a complex aggregate. It is now at least self-evident that the possibil- 

 ity of chance variations having anything to do with determining the mode 

 of evolution of the tail of fishes is in the highest degree improbable. 

 Extreme reduction and degeneration in certain directions have also 

 been clearly shown to occur, the evidence on this point being too pa!i)a- 

 ble and conclusive to admit of dispute. 



Not less weighty than the objection urged in the case of the MolocephaU 

 is that which might be urged in the case of Caturus and Leptolepls, 

 Liassic and Oolitic forms, almost as outwardly homocercal and struct 

 urally almost as heterocercal as the most differentiated existing Teleosts. 

 Why is it, it might be inquired, that these forms have reached hetero- 

 cercal and outwardly homocercal specialization so soon ? Or w\\y should 

 the archaic or primitive diphycercal type be preserved in the existing 

 Dipnoi if these and their kindred have been subject to the same outward 

 mechanical forces as the line which ended in the i)roduction of the ex- 

 treme heterocercy of the Teleosts ? Have t\\B Dipnoi, however, developed 



