162 THE AMERICAN NATURALIST [Vol. XLIX 



each generation starts with a slightly increased capital 

 and carries on its development a little further under in- 

 ternal conditions modified accordingly. 



At this point it may be asked, Is the response to en- 

 vironmental stimuli a purely mechanical one, and, if so, 

 how can we account for the fact that at every stage in 

 its evolution the organism is adapted to its environment? 

 We shall have to return to this question later on, but it 

 may be useful to point out once more that there is good 

 reason to believe — especially from the experimental 

 work of Jennings — that the response of even a unicellu- 

 lar organism to stimuli is to a large extent purposive; 

 that the organism learns by experience, by a kind of 

 process of trial and error, how to make the response most 

 favorable to itself under any given change of conditions ; 

 in other words, that the organism selects those modes 

 of response that are most conducive to its own well- 

 being. Under the term response to stimuli we must, of 

 course, include those responses of the living protoplasm 

 which result in modifications of bodily structure, and 

 hence the evolution of bodily structure will, on the whole, 

 be of an adaptive character and will follow definite lines. 

 There is good reason for believing, however, that many 

 minor modifications in structure may arise and persist, 

 incidentally as it were, that have no significance as adap- 

 tations. 



One of the most remarkable and distinctive features 

 of the lower vertebrates is the presence of gill-slits as 

 accessory organs of respiration. These gill-slits are 

 clearly an adaptation to aquatic life. When the ances- 

 tors of the higher vertebrates left the water and took to 

 life on land the gills disappeared and were replaced by 

 lungs, adapted for air-breathing. The change must, of 

 course, have been an extremely gradual one, and we get 

 a very clear indication of how it took place in the sur- 

 viving dipnoids, which have remained in this respect in 

 an intermediate condition between the fishes and the 

 amphibia, possessing and using both gills and lungs. 



