Pag 



E FOUR 



E \- O L U T I N 



June, 1930 



tion, or difference arising between an offspring and 

 its parent, must have been due to some definite con- 

 dition or stimulus, within or surrounding that par- 

 ent, whether we can at present trace it or not. A 

 repetition of this condition then would bring forth 

 a similar variation again. In some quarters it is 

 added further that such causation must therefore 

 tend in itself to explain the course of evolution. 

 This in turn would seem to circumvent the necessity 

 of invoking "natural selection" to do more than 

 help out in a secondary and occasional fashion. 

 For it is often said it feels "' philosopliicaUy unsatis- 

 fying" to believe that all the order and organization 

 of living things could have come about through such 

 a chance process as natural selection admittedly is. 

 It is evident that a real decision of the questions 

 at issue can be reached only on the basis of real 

 data regarding the nature of those differences which 

 distinguish one generation of individuals from its 

 predecessors, and which they in turn tend to trans- 

 mit as a heritage to their descendants. Tiiat is, 

 we must not remain content to view evolution from 

 afar, but must view close up, as through a micro- 

 scope, the transitions now occurring out of which tlie 

 evolutionary story is pieced together. Tlie science 

 which essays this study is "genetics." 



///. GENETIC PRINCIPLES REVIEWED FOR 

 THE NON-BIOLOGIST 



During the present century genetics, building 

 upon the earlier discoveries of Mendel, has practi- 

 cally solved the problem of the method of inheri- 

 tance of the differences referred to, once they have 

 arisen. All modern genetic work converges to show 

 that the heritable differences between parent and 

 offspring, between brother and sister, in fact, be- 

 tween any organisms which can be crossed, have 

 their basis in differences in minute self-reproduc- 

 ing bodies called genes, located in the nucleus of every 

 cell. The genes themselves are too small to be 

 separately visible, but hundreds or thousands of 

 them are linked together into strings, and these 

 strings of genes, together probably with some ac- 

 cessory material, are large enough to be seen through 

 the microscope by the cytologist ; they constitute 

 the sausage-shaped bodies called chromosomes. We 

 know that, ordinarily, each individual gene in a 

 string is different from every other gene in the 

 same string, and has its own distinctive role to play 

 in the incomparably complicated economy of the 

 cell. Moreover, the genes in different chromosomes 

 are different from one another, except in the case 

 of homologous or twin chromosomes, i.e., the corre- 

 sponding chromosomes which each cell of an individ- 

 ual received from the father and from the mother of 

 the individual, respectively. To match each chromo- 

 some that was derived from your father, every cell 

 of you has in it also a similar chromosome ( though 

 not necessarily quite identical) derived from your 

 mother, so that it contains in all two complete sets 

 of genes . The proper functioning of the cell dur- 



ing its life dejiends upon the proper cooperative 

 functioning of its thousands of different genes. 



Each given gene in the cell must of course have 

 its own specific chemical composition, differing from 

 gene to gene, though there is no doubt a chemical 

 relationsiiip between all genes. As yet, however, 

 we have no knowledge as to what the chemical com- 

 position of any individual gene, or of genes as 

 a group, is. Whatever it is, we can not escape the 

 fact that the different genes, through differing chem- 

 ical reactions with other substances in the cell, pro- 

 fluce by-products which have a very profound in- 

 fluence upon the properties of the protoplasm. 

 And through the combined influences of all the chem- 

 ical products of the thousands of different genes 

 in a cell, meeting one another in the common pro- 

 toplasm and then interacting in devious ways to 

 form further products again, the exact form and 

 physical and chemical characteristics of all parts of 

 the cell that contains those genes will be determined, 

 for any given set of outer conditions. Changing 

 conditions external to the cell will of course change 

 the properties of the protoplasm too, but what 

 form and behavior it can and will show for a given 

 set of outer conditions depends primarily upon what 

 genes it has. And since the body of a man or other 

 animal, or a plant, is made up of its cells, and the 

 form and other properties of that body depend 

 upon the properties of these constituent cells — their 

 foiTn, the way they fit together and work — it is 

 evident that, less directly but no less surely than 

 in the case of the individual cells, the characteristics 

 of the whole body depend upon the nature of the 

 genes in the individual cells. 



These individual cells of the body have, during 

 the development of the embryo, been derived from 

 the original fertilized egg cell, through a succession 

 of cell divisions in the course of each of which every 

 chromosome and every gene present in the dividing 

 cell also divided in half, one half of every chromo- 

 some and gene then entering one of the two daughter 

 cells and the other half entering the other daughter 

 cell. Between divisions the chromosomes and genes 

 usually had a cliance to grow back to their original 

 size. Thus it results that everj' cell of the body has 

 the same kinds and numbers of chromosomes and 

 genes as the fertilized egg had, and as every other 

 cell in the body has. The original two sets of genes 

 of the fertilized egg — one set received from the sperm 

 of the father, the other similar set derived from what 

 the egg of the mother contained before fertilization 

 — are still both present in every cell of you. But 

 these two sets of genes of the fertilized egg were all, 

 and more, than were needed to result in a complete 

 man. We see, then, that every single cell of you, in 

 the skin, tlie brain or anywhere else, contains the ^ 

 makings of a complete man or woman, and that you 

 are in this sense wrapped up within yourself many 

 trillion fold. Not each cell may grow up into an 

 entire man, of course, but must remain content to 

 do its specialized share, even though it lias a full 

 (Continued on page 14) 



