The Modern Idea of Evolution 



17 



abandon this view. The animal itself 

 is not the unit which varies; the germ- 

 plasm of the animal is what varies, and 

 the animal (or plant) is merely the 

 product of (hypothetical) germinal fac- 

 tors. The importance of this change 

 in viewpoint may not be apparent at 

 first sight, but in practice it is found to 

 be weighty, for it substitutes precision 

 and clarity of thought for a wholly 

 mystical idea, and enables us to breed 

 understandingly . 



Morgan quotes many instances of 

 variation in the fruit fly Drosophila, 

 on which his work for the last ten years 

 has been mostly based. It is a variable 

 creature; if we catch a bottle full of 

 the same species, we will find that in 

 details they offer numerous differences, 

 and if we go on to breed them we will 

 find that many of these differences are 

 passed on to their progeny. These 

 differences are further found to be 

 inherited in what is known as Men- 

 delian fashion, with which the reader 

 is doubtless familiar. 



The point of interest is that every 

 once in a while a fly is born with some 

 character different from the correspond- 

 ing character of his parents. The par- 

 ents had normal wings, for example, the 

 offspring comes into the world with 

 extra long wings, and this extra length 

 is found to be inherited in a very defi- 

 nite way. 



It is fiirther discovered as the result 

 of long, careful, and widespread ob- 

 servation (Morgan and his associates 

 and students have bred more than half 

 a million flies altogether) that with this 

 change in wing-length go certain other 

 changes in the fly. A change, a varia- 

 tion, seems never to affect one part of 

 the fly alone ; it has an indefinitely large 

 nimiber of effects in various parts of 

 the body (and, must we not suppose, 

 on the "mind," too?). 



To be strictly accurate , then, we should 

 not say that a certain variation affects 

 length of wing, but that its chief effect 

 is to lengthen the wing. ' ' For example, a 

 mutant stock called rudimentary wings 

 has as its principal characteristic very 

 short wings. But the factor for rudi- 

 mentary wings also produces other 



effects as well . The females are almost 

 completely sterile, while the males 

 are fertile. The viability of the stocks 

 is poor. When flies with rudimentary 

 wings are put into competition with 

 wild flies relatively few of the rudi- 

 mentary flies come through, especially 

 if the culture is crowded. The hind 

 legs are also shortened. All of these 

 effects are the results of a single factor- 

 difference. 



BY-PRODUCTS OF VARIATION 



"One may venture to guess that some 

 of the specific and varietal differences 

 that are characteristic of wild t^^pes and 

 which at the same time appear to have 

 no survival value, are only by-products 

 of factors whose most important effect 

 is on another part of the organism where 

 their influence is of vital importance." 



" I am inclined to think that an over- 

 statement to the effect that each factor 

 may affect the entire body, is less 

 likely to do harm than to state that each 

 factor affects only a particular character. 

 The reckless use of the phrase 'unit 

 character ' has done much to mislead the 

 uninitiated as to the effects that a single 

 change in the germ-plasm may produce 

 on the organism. Fortunately, the ex- 

 pression 'unit character' is being less 

 used by those students of genetics 

 who are more careful in regard to the 

 implications of their terminology." 



Now to Sinn up the new view of the 

 problem of the nature and origin of 

 variations, which Darwin failed to 

 solve. As to the actual cause of these 

 changes in the germ-plasm, we know no 

 more than he did; we suppose them to 

 be chemical reactions. But as to their 

 effects, as to what variation actually 

 means, we have learned a great deal. 

 We have found out that the germinal 

 differences of an individual are in- 

 herited separately from each other ; that 

 every change in the germ-plasm — i. e., 

 every variation — affects not one but a 

 large nimiber of characters; and con- 

 versely, of course, that every visible 

 character is the resiilt of the concurrent 

 action of a large number of factor- 

 differences or variations. It has like- 

 wise been demonstrated in many cases 



