AQUATIC MAMMALS 



The reader will doubtless discover in die following pages that the 

 writer subscribes to a modification of the Lamarckian theory, or theory 

 of the inheritance of acquired characters, perhaps better expressed as 

 inheritance based upon the use or disuse of parts. This theory has 

 fallen into disfavor for the reason that no one has been able to demon- 

 strate it, but that is no reason for ignoring its possibility. The failure 

 to prove it doubtless lies in the probability that it operates too slowly 

 to be discernable in the lifetime of one man. There have been futile 

 experiments such as the one in which the tails of white mice were re- 

 moved throughout several generations in the expectation of producing 

 a race of bobtails. Naturally this was foredoomed to failure under 

 any circumstances for account was not taken of the fact that amputation 

 of the tail did not remove the mouse's need for a tail as a balancing 

 organ; or that the germ plasm remained unaltered. But if we could 

 cause white mice to dig industriously in the ground for several hours 

 each and every day, and oblige all their descendants to do likewise with 

 conscientiousness, then would there be produced, according to my 

 belief, a race of mice with feet much more specialized for digging: not, 

 emphatically in twenty or a hundred generations, but in twenty or a 

 hundred thousand generations. 



In a study of this kind one frequently encounters the puzzling condi- 

 tion illustrated by the following situation. In the rorquals the narrow 

 part of the tail — the peduncle — has built up a sharply ridged keel, both 

 above and below, of fibrous tissue which acts to reduce water resistance 

 during swimming. The broad part of the tail — the flukes — in this 

 animal has built up precisely the same sort of fibrous expansions, but 

 for the purpose of increasing water resistance. Thus the same sort of 

 structure has been developed in response to stimuli which appear dia- 

 metrically opposite. This is entirely beyond our comprehension, and if 

 the condition is referred to at all it is usually in the most cautious, 

 evasive terms. All that can be told is that often a structure will see^n 

 to develop where there has clearly become a need for it without our 

 being able to distinguish any really directive stimulus at all, in spite 

 of the fact that logic dictates that a new modification cannot anticipate 

 a new function. 



In considering the specialization exhibited by aquatic mammals one 

 must constantly bear in mind that the results seen depend upon the 

 strength of the stimuli involved, the capacity of the animal to respond 

 to them, the strength of possible antagonistic stimuli, and the length 

 of time involved. The aquatic evolution of one particular type of 



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