12 GENETICS AND EUGENICS 



course of a few generations the only individuals left alive 

 will be those that have developed it, while those that did 

 not develop it will be exterminated by the law of natural 

 selection." 



This theory stands today in the main as Darwin left it, 

 the chief advances since his time being concerned with one 

 or other of the two factors, variation and heredity, concern- 

 ing which our knowledge, though still incomplete, has made 

 notable advances. But before we pass to the consideration 

 of these, let us pause to inquire what were the lines of 

 evidence upon which Darwin relied to establish his theory. 



These have been well summarized by T. H. Huxley (1825- 

 1895) who by his able championship of Darwin's views did 

 more than any other one man to gain for these views general 

 recognition and acceptance. As modified by Lock, Huxley's 

 summary is as follows : — 



" 1. The Gradation of Organisms, Both in the animal and 

 vegetable kingdoms we may trace, in spite of certain gaps, a 

 long series of gradations in complexity of structure, so that 

 between the simplest and the most complicated of living 

 things a great number of intermediate stages are to be found. 

 When we pass to the lower end of the scale in either case, we 

 come upon a group of creatures of comparatively simple 

 organization. Among them we find members with regard 

 to which we cannot definitely say that they are either animals 

 or plants. Moreover, these unicellular organisms resemble 

 in many ways the egg-cell from which every individual among 

 the higher animals and plants originates. 



"2. Embryology. All the members of a particular group of 

 animals or plants as a rule resemble one another more closely 

 in the early stages of their individual development than they 

 do in the adult condition, and in the earliest stages of all they 

 are often indistinguishable. These facts are explained if we 

 suppose that such individuals have a common origin, that 

 they are descended from a common ancestor, and that traces 

 of their pedigree are still to be observed in the developmental 

 stages through which each one passes. We do not find a com- 



