Various Modifying Agencies. 135 



eumstance which would cause the water to become salted, would inevit- 

 ably change all the animal life involved, or perhaps destroy it entirely 

 of both of which Geology furnishes many examples. 



Mechanical Effect of Water. 



When we come to discuss the osseous structure of vertebrates, we 

 shall see how habitual strains develop particular parts. We have an 

 illustration of this in the development of organs in flowing currents. 

 Currents constantly cause involuntary migration in very many aquatic 

 animals, sweeping them into new environments, where they are either 

 destro3 T ed or modified to suit their new environment. This happens 

 particularly to animals without organs. On the other hand, all animals 

 with swimming organs have them developed or adapted to contending 

 against the currents. An organ used for beating against the water will 

 naturally tend to take the form of a fin, since that form combines light- 

 ness, strength and flexibility, with a minimum of friction and maximum 

 of surface. Natural selection would inevitably pick out such an organ 

 for swimming. And no matter what shape the organ may be, if it be 

 put to work in the water it begins to assume the character of a fin. 

 There can be no doubt that the first limbs were of the nature of fins, 

 which were modified from folds or ridges of the integument, on the 

 back, belly, tail and sides. From fins the limbs of reptiles and land 

 vertebrates were gradually modified, and whenever any land vertebrate 

 has found it advantageous to again have recourse to the water, his limbs 

 have been modified backward toward the model of the fins. Ducks, 

 Cormorants, &c. , use their wings both for flying and swimming. But 

 the Penguin uses his for swimming only, and so they have actually been 

 modified from wings into fins not yet as complete or as shapely as those 

 of fishes but still quite effectual. The feathers have become very like 

 scales ( See fig. 74 ). Crocodiles, Turtles, the Ornithorhynchus, Otter, 

 &c. , have webbed feet. The limbs and hind end of the bodies of the ma- 

 rine mammalia, Whales, Seals, Dolphins, Dugongs, &c. , have all been 

 modified from the forms of land mammals toward those of fish. That 

 such modification has taken place, is unquestionable, and it is both proof 

 and illustration of the sort of reaction which is provoked in the vertebrate 

 organism by the dynamic agency of water. The limbs of those insects, 

 crustaceans and worms which ha\e become aquatic, are modified and 

 adapted more or less completely for swimming. And all these swim- 

 mers are in a constant struggle, in maintaining themselves against cur- 

 rents or in moving themselves in pursuit or retreat, the weaker con- 

 stantly lost in the struggle and the strong becoming stronger from gen- 

 eration to generation. 



A great many marine animals that are sedentary in their habits, have 

 organs for holding on to some rock or other object, so as to keep from 



