PRESENT PROBLEMS OF PALEONTOLOGY. 231 



only assignable cause; subsequently I realized that it was not explain- \ 

 able by the Lamarclrian hypothesis. 



I then attributed it to an unknown law of evolution, and there 

 I believe it rests to-day, namely: as a process of which we do not 

 know the cause. Still more recently, however, comes the discovery that 

 original kinship is partly at least a control-principle. For example, 

 in the descent of independent stocks of hornless animals arising 

 from a common stock, rudimentary horn cores are found to appear in- 

 dependently in exactly the same region of the skull, indicating a kind 

 of predetermination in the stock, or potential of similar evolution. The 

 facts on which this law of mutation, properly called, rests have been 

 misunderstood, totally denied, or explained away by selectionists as 

 survivals of favorable out of indiscriminate variations. Even my col- 

 league, Scott, has identified these phenomena with the saltations of De S 

 Vries. Nevertheless, I regard the genesis of new adaptive characters 

 from almost imperceptible beginnings as a vera causa, and as one of the 

 greatest problems we have to solve. 



That a natural solution will be found goes without saying, although 

 this principle, as stated, is undoubtedly of a teleological nature. Its 

 philosophical bearings are of far reaching importance. Just as we 

 demand a continent to transfer land animals from Australia to South 

 America, so we demand a natural law to explain these facts. 



The creative factors of fitness cooperating with selection, which, in 

 my judgment, are now well demonstrated, reside either primarily in the 

 environment, in the bodies of animals, or in the germinal cells — they 

 all ultimately find their way into the germinal cells. They may be 

 summarized as follows: 



1. Segregation. — Besides the familiar geographical segregation of 

 animals, which reaches its highest expression in insular forms, such as 

 the pygmy fossil elephants of Malta and those recently discovered in 

 Cyprus (Wade), there is the no less effective segregation of habit among 

 animals existing in the same geographical regions and under the same 

 climatic conditions, but seeking different varieties of food on different 

 kinds of soil. These give rise to what I have called local adaptive 

 radiations, a principle which explains the occurrence in the same 

 country, and almost side by side, of very conservative as well as very 

 progressive forms. 



2. Adaptive Modification. — This is a plastic principle which tends 

 in the course of life to an increasing fitness of the bodies of individuals 

 to their special environments and habits, well illustrated among men 

 in the influence of various trades and occupations and operating both 

 in active and in passive structures. Consistent with the adaptive modi- 

 fication principle is the fact that every individual requires habit and 

 environment to model it into its parental form; and in every change 

 of environment or habit every individual is carried an infinitesimal 



