MULLER 105 



a specific pheno?ne?ion, a summary of our knowledge concenmig it, 

 and then indicatioji of the pathways that 7night prove fruitful for 

 future research. This type of contribution is know7i as a '''review 

 paper,'''' and, whe'n properly done, serves as a primary tool used by 

 biologists to keep up with progress in fields other than their own. 

 Midlefs paper not o?ily provided this service for biologists of his 

 time, but also statids as a valuable reference and a classic surmnary 

 for anyone interested i7i the developmejit of the theory of the gene. 

 The validity of Midler's remarks concerfiing future research and its 

 aims is highlighted by his paper published five years later, i?i which 

 he announced successful experimental mutatio?i (see p. 149). 



x. the relation between the genes 



and the characters of the 



organism 



The present paper will be con- 

 cerned rather with problems, and the 

 possible means of attacking them, than 

 with the details of cases and data. The 

 opening up of these new problems is 

 due to the fundamental contribution 

 which genetics has made to cell physi- 

 ology within the last decade. This 

 contribution, which has so far scarcely 

 been assimilated by the general physi- 

 ologists themselves, consists in the 

 demonstration that, besides the ordi- 

 nary proteins, carbohydrates, lipoids, 

 and extractives, of their several types, 

 there are present within the cell thou- 

 sands of distinct substances— the 

 "genes"; these genes exist as ultra- 

 microscopic particles; their influences 

 nevertheless permeate the entire cell, 

 and they play a fundamental role in 

 determining the nature of all cell sub- 

 stances, cell structures, and cell activ- 

 ities. Through these cell effects, in 

 turn, the genes afi^ect the entire organ- 

 ism. 



It is not mere guesswork to say that 

 the genes are ultra-microscopic bodies. 

 For the work on Drosophila has not 

 only proved that the genes are in the 

 chromosomes, in definite positions, but 

 it has shown that there must be hun- 

 dreds of such genes within each of the 



larger chromosomes, although the 

 length of these chromosomes is not 

 over a few microns. If, then, we divide 

 the size of the chromosome by the 

 minimum number of its genes, we find 

 that the latter are particles too small 

 to give a visible image. 



The chemical composition of the 

 genes, and the formulae of their reac- 

 tions, remain as yet quite unknown. 

 We do know, for example, that in 

 certain cases a given pair of genes will 

 determine the existence of a particular 

 enzyme (concerned in pigment pro- 

 duction), that another pair of genes 

 will determine whether or not a cer- 

 tain agglutinin shall exist in the blood, 

 a third pair will determine whether 

 homogentisic acid is secreted into the 

 urine ("alkaptonuria"), and so forth. 

 But it would be absurd, in the third 

 case, to conclude that on this account 

 the gene itself consists of homogentisic 

 acid, or any related substance, and it 

 would be similarly absurd, therefore, 

 to regard cases of the former kind as 

 giving any evidence that the gene is 

 an enzyme, or an agglutinin-like body. 

 The reactions whereby the genes pro- 

 duce their ultimate effects are too 

 complex for such inferences. Each of 

 these effects, which we call a "char- 

 acter" of the organism, is the product 

 of a highly complex, intricate, and 

 delicately balanced system of reac- 

 tions, caused by the interaction of 



