INHERITANCE 301 



a large number of characters into line with the principles pre- 

 sented. Thus stature, proportions of the parts of the body, build, 

 as well as nearly all of the physiological and mental characteristics 

 in Man are evidently dependent upon multiple genes. In certain 

 Fruit Flies eye color may be influenced by more than forty pairs 

 of genes and the wings by upward of ninety. Thus it is becoming 

 increasingly clear that what a given gene will produce is determined 

 by the constitution of the gene plus its interaction with many, 

 if not all of the other genes of the complex, although, of course, 

 as we have seen, the single gene pair, in many or most cases, does 

 have its most conspicuous effect on a certain character of the 

 organism. And, furthermore, what the gene complex will produce 

 bears an intimate relationship to the environment. For instance, 

 in Fruit Flies the abnormal condition of extra legs is inherited in 

 typical Mendelian manner when the flies are reared at a low 

 temperature; whereas supernumerary legs do not appear in flies 

 with the same gene heritage when bred at a higher temperature. 

 In short, the environment, in certain cases at least, may act as 

 a differential intensifying or diminishing gene action. 



So it happens, as is usually the case, the more a problem is stud- 

 ied the more complex it appears to become. Suffice it to say that 

 although our knowledge of inheritance is to-day very much broader 

 than Mendel conceived on the basis of his classic experiments, it 

 is evident that he supplied us with basic principles which are 

 affording a common denominator for an ever-increasing number of 

 facts in genetics. 



D. Mechanism of Inheritance 



With this general outline of genetic principles before us, it is 

 now necessary to bring them into relation with the facts so far 

 discovered in regard to the structure of the germ cells. In other 

 words, we have assumed, on the basis of the experimental results 

 derived from breeding plants and animals, the existence of genes, 

 the occurrence of segregation, etc., but has the actual study of 

 cells (cytology) by means of the microscope given any evidence 

 of the physical basis of genes and of a segregating mechanism? 

 The reader will at once answer this in the affirmative on the basis 

 of our discussion of the origin and structure of the germ cells and 

 their behavior in fertilization. Accordingly the essential facts may 

 now be restated from this viewpoint. (Fig. 165.) 



