'AGE FOURTEEN 



EVOLUTION 



June, 1930 



(Continued from page 4) 

 cargo of genes, because its structure and activities 

 are limited and regulated in various ways through 

 the mutual influences received from the other cells 

 in the body. The various cells of different organs 

 developed differently from one another because, 

 though possessing the same genes, they found them- 

 selves in different influences, from the start. Only 

 the germ-cells, then, may eventually realize anything 

 like their full potentialities. 



How We Differ From One Another 



All this explanation, somewhat off the main theme, 

 may serve to furnish some sort of notion of how the 

 characteristics, in fact the entire substance, of any 

 human or other living being depend upon its genes, 

 acting in a chemically coordinated fashion. So com- 

 plicated is the manner in which the products of the 

 different genes react with one another that no final 

 product and no characteristic of the adult body is 

 due to any one specific gene, but in the production 

 of every organ, tissue or characteristic, numerous 

 genes take part. Nevertheless, if one individual 

 differs from another individual in regard to just 

 one of the genes that do take part, it will be seen 

 that the given characteristic in the two individuals 

 will be different, and so, conversely, a difference 

 between two individuals in regard to a certain char- 

 acteristic, let us say eye color, may be due to a dif- 

 ference between just two given genes in them rather 

 than other genes. 



By studying the characteristics that appear 

 among the descendants in later generations, after 

 individuals differing in regard to one or more genes 

 have crossed together, the definite Mendelian laws 

 and the laws of linkage governing the inheritance 

 of genes have been determined, and they are found 

 to have a practically universal validity. There is 

 no use attempting here to formulate in detail these 

 rules and their working out ; this usually occupies 

 the major portion of a course in genetics. It is 

 sufficient to call attention to a few pertinent facts. 



When two germ-cells tliat differ in respect to a 

 certain gene, e.g., the egg having the gene for brown 

 and the sperm that for blue eyes, fertilize each other, 

 neither gene is lost, but the resulting individual 

 possesses both genes in every one of his cells, even 

 though his eyes may show preponderantly the brown 

 color, brown being said to be the dominant gene 

 and blue the recessive. Half the germ-cells formed 

 by an individual of such mixed composition will 

 carry the brown gene and not tlie blue one, the rest 

 carry the blue gene and not the brown, and so there 

 is as good a chance for any one of his children to 

 inherit the blue gene as the brown one. Moreover, 

 it is found that neither the blue gene nor the brown 

 one, when inherited by the next generation, shows 

 any weakening or other trace of its former sojourn 

 with a gene of opposite character. It persists 

 through the generations uncontaminated by its as- 

 sociate-genes. 



Now if the individual of dual composition had 

 also been dual in regard to some other kind of gene, 

 say for hair shape, having received tlie gene for 

 curly hair from the parent that gave him brown 

 eyes, and the parent that donated to him the gene 

 for blue eyes, then, although he would probably 

 show moderately curly hair, nevertheless, as in the 

 case of the eye colors, half of the germ-cells which 

 he produced would carry curly and the rest would 

 carry straight. However — and here lies the import- 

 ant point — not all those germ-cells of his that car- 

 ried brown would be the ones carrying curly, nor 

 would all those carrying blue carry straight ; there 

 would be some carrying the combination, new per- 

 haps for this family, of brown eyes with straight 

 hair, and other germ-cells carrying the converse new 

 combinations — blue eyes with curly hair. Thus, 

 by crossing and recrossing individuals with different 

 genes will be produced in subsequent generations un- 

 til all possible combinations of the differing genes in 

 crossable individuals have been given a trial. Oc- 

 casionally totally new effects, new characteristics, 

 are thereby produced, when some new combination 

 is obtained through the unexpected interaction of 

 genes which never before had been tried out to- 

 getlier, but such cases are rare. Were the process 

 just described the only method of obtaining change 

 in the characteristics of organisms, evolution would 

 soon reach its limit. i 



How Species Arise 



On analyzing tlie results of various crosses, it is 

 found, as might have been expected, that closely 

 related individuals differ in more genes (in fact, 

 in about twice as many as do brothers or sisters), 

 individuals of different races differ in still more 

 genes, and those of different species, where the 

 crosses can be made and analyzed, are found to 

 differ in a j'et greater number. In each case, how- 

 ever, the differences are of the same general nature 

 — differences in ordinary genes in the chromosomes, 

 tliat normally are handed down in heredity in the 

 orthodox Mendelian fashion. In fact, in some cases, 

 as Baur and others have found, the difference be- 

 tween two members of a family may be in regard to 

 the very genes which also distinguish two different 

 species as a whole. The species-differences then, 

 are simply more abundant, and have liad a chance 

 to become more select(ed). And same is true of the 

 differences between more widely separated groups 

 (with the further proviso, that the number and 

 arrangement of the genes, as well as their kind, 

 may be difl'erent). For, in the course of evolution, 

 as one species changed into another and then again * 

 into another, it would gradually diverge so far 

 from its progenitors as to be placeable in a separate 

 genus, then in a separate family, order, class, etc., 

 although all along differences would be accruing in 

 the same manner as before. If they can accrue in 

 any other manner we should by this time have got- 

 ten evidence of it. 



