CHAPTER IV 



THE FUNDAMENTAL POSTULATE UNDERLYING 

 ALL EVIDENCES OF EVOLUTION 



Every science rests in last analysis upon certain postulates or justi- 

 fiable assumptions, certain verified or verifiable truths that must be 

 admitted before any progress can be made in gaining a further under- 

 standing of the content of that science. Geology, for example, must 

 assume as valid the dynamical laws of Newton and the law of gravita- 

 tion, as well as basic laws of chemistry. Biology assumes the validity 

 of the laws of physics and chemistry, for biology is the fundamental 

 science of the transformations of matter and of energy in living matter; 

 but, in addition, there are also some biological postulates that seem 

 to be so well established that they have come to be thought of as 

 truisms. 



One of the truisms of biology is the familiar fact that like produces 

 like. How surprised one would be if sparrows had anything but spar- 

 rows for offspring, or if two Caucasic parents were to have a Negro 

 child ! Now, a careful survey of the situation reveals the fact that the 

 only postulate the evolutionist needs is no more nor less than a 

 logical extension of what the layman considers a truism or a self-evi- 

 dent fact, namely, that fundamental structural resemblance signifies 

 genetic relationship; that, generally speaking, the degree of closeness of 

 structural resemblance runs essentially parallel with closeness of kinship. 

 Most biologists would say that this is not merely a postulate, but 

 one of the best-established laws of life. However obvious the validity 

 of this postulate may be, it is the plain duty of one who attempts to 

 justify the evolutionary principle to avoid taking any steps that are 

 open to the least bit of valid criticism. If we cannot rely upon this 

 postulate, which may be called the principle of homology, we can 

 make no sure progress in any attempt to establish the validity of 

 the principle of evolution. 



The postulate we are now discussing is tantamount to an affirma- 

 tion of the fact of heredity. We rely upon this fact in our everyday life. 

 When we plant a certain kind of seed we expect to get a certain kind 

 of plant; when we breed a certain kind of dog we expect offspring 



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