i44 TEE POPULAR SCIENCE MONTHLY 



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an immense barrier against the penetration of the blond races into the 

 tropics. 81 



Summarizing in a word the results of these studies on man, we may 

 say that the death rate is unquestionably selective. There are still those 

 who assert that while natural selection applies to the lower organisms 

 its force is nil in civilized society. Against such a view the evidence 

 of biometric workers seems fairly conclusive. But concernng the way 

 in which this selective death rate occurs we know lamentably little. 

 Indeed, the whole problem of the basis of natural selection in man is 

 open to investigation. The biometric work which has been done shows 

 how complex the whole problem is, and how idle to attempt its solution 

 by any means but the analysis of large masses of carefully collected 

 data by refined statistical methods. 



III. Supplementary Tests of Fitness 



The capacity of an individual for survival is doubtless dependent 

 upon the fitness of its several organs for performing their respective 

 functions, or upon the nicety of their coordination. At present, the 

 ultimate goal of investigations of natural selection would seem to be 

 the determination of the significance for survival of each deviation 

 from type of as many organs or characteristics as practicable. Upon 

 the evidences afforded by a comprehensive series of investigations of 

 this kind must depend our final views concerning the significance of 

 natural selection as a factor in organic evolution. 



Fitness may be tested in various ways. A series of individuals may 

 be actually subjected to a struggle for existence — be " exposed to risk," 

 to use an actuarial term — and the difference between the series of indi- 

 viduals which survive and those which perish measured in terms of 

 biometric constants. This is essentially the course followed in the 

 studies reviewed in the preceding paragraphs. It is from the stand- 

 point of the evolutionist the most direct method. 



Fitness may, however, be tested in some favorable cases in which 

 the individual lays down a series of organs (with measurable charac- 

 teristics) only a portion of which may develop to maturity. Here one 

 may find that the elimination of organs within the individual is not 

 random, but selective. A comparison of the characteristics of the 

 organs which fail with those which complete their development may 

 furnish information as to the characteristics which make for fitness or 

 unfitness for survival. 



Again, physiological criteria — e. g., efficiency in the maturing of 



11 One should also read the most interesting chapter on the problem of the 

 white man in the tropics in Ripley's "Races of Europe." There, structural 

 characteristics as well as pigmentation are considered. 



