Page:Popular Science Monthly Volume 84.djvu/148

144 an immense barrier against the penetration of the blond races into the tropics.

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 concernngconcerning [sic] 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.

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 individuals 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 standpoint 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 characteristics) 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