EVIDENCES— EXISTING ORGANISMS 117 



of some species, and that the duration of experiments has been an 

 infinitesimal part of this period. The mere fact that even indi- 

 viduals are capable of some degree of adaptive adjustment leads 

 one to believe that the species which they constitute, since their 

 fundamental qualities are rooted in adaptations, are also plastic 

 in that respect. But since the species is a more permanent unit 

 than the individual, it is not difficult to understand its obviously 

 slighter susceptiliility to change. 



Interspecific Relationships. Interspecific relationships among 

 organisms offer another convincing proof that such change has 

 occurred. According to the doctrine of special creation, a succes- 

 sion of forms appeared ranging from simple to complex, with man 

 as the last. It is not surprising, therefore, according to this or 

 any other view, that many lesser beings were in existence when 

 man and related mammals appeared. Parasitic species, however, 

 of many phyla are dependent upon the human race for food. If 

 all of these were created in a brief span of time, granted that they 

 could exist until the creation of man, it has been suggested that 

 Adam and Eve must have l^een l)eset with all of the creatures that 

 now live on or in the human ])ody. Such a ludicrous picture is 

 contrary to reason. Obviously change in the ancestors of these 

 organisms has led to their association with man since his appear- 

 ance on earth. If independently created, there is, as already 

 pointed out, no reason to expect them to resemble so closely other 

 less specialized forms. 



Cause and Effect in Variation. One other phase of organic 

 change is that of so-called fortuitous variation. It is doubtful that 

 any variation occurs independently of response to definite condi- 

 tions, but a complex organism consists of many parts, any one of 

 which may vary. The entire body of the organism is coordinated, 

 and each part through its functions affects other parts. This is a 

 primary result of specialization, but it is conspicuous in such effects 

 as the endocrine glands produce. Persistence of the thymus results 

 in gigantism; thyroid insufficiency is responsible for cretinism; 

 removal of the gonads results in the absence of the normal second- 

 ary sexual characters. The over-development of parts, or their 

 failure to develop, is in all cases a definite response to a definite 

 condition. When, as in these cases, a condition appears without 

 evident relation to the environment, it is said to be fortuitous. 



While a stable condition persists, there is no reason to expect a 



