EVOLUTION TODAY 435 



durinp; growth is in no sense an adaptation to life in sunlight. 

 These conditions are, however, no less a result of environmental 

 stimulus than tanning. With respect to adaptation, all changes 

 appearing in response to the internal environment must be 

 incidental. 



Incidental changes are a part of the organism just as much as 

 adaptive modifications, hence they are availa])le for other adjust- 

 ments to the environment. Any incidental modification when it 

 is once developed may affect the organism's relations to other 

 conditions than those which brought about its development. 

 Normal nervous development is partly a response of the heritage 

 to normal thyroid secretion, yet it accomplishes most of our adjust- 

 ments with the conditions under which we live. 



How Has Evolution Progressed? When we examine the 

 phylogenetic series worked out by paleontologists we are able to 

 learn something of the superficial aspects of past evolution, even 

 though we may not be able to fathom the underlying forces. We 

 are al)le to see evidences of the heritage of long series at different 

 stages of development. Correlation of these characters with the 

 environment may be deduced from a comparison with similar 

 structures of living organisms, but a very definite indication of 

 past environmental conditions, such as climate, is afforded by the 

 fossil remains of other organisms, especially the green plants. 

 We can therefore learn from paleontology not only how the heritage 

 of a given hne of descent has changed from age to age but also 

 under what conditions these changes have taken place. 



An examination of living organisms is of great value in connec- 

 tion with this study, for even in living things we find conditions 

 similar to those which have preceded evolution in the past. 

 From time to time in the previous chapters of this book organisms 

 have ]jeen mentioned which show a distinct capacity for evolu- 

 tionary change. 



The Heritage. Whether in extinct organisms w^hose descend- 

 ents are before us or in living things whose possibihties can only 

 be estimated, we find that the heritage must not be limited rigidly 

 to one type of response if evolution is to occur. We cannot take 

 a trout out of water and expect it to live, much less to become a 

 terrestrial animal, but fishes have existed and still exist which 

 might well accomplish this transition in some degree. Fishes with 

 functional lungs can breathe air or secure their oxygen from the 



