SOME ASPECTS OF ZOOLOGY 49 



our evidence is than it was only fifteen to eighteen years 

 ago. 



I said just now that there must be in the germ-plasm a 

 " factor " for every separate character exhibited by the 

 individual body or soma. What do we know about these 

 " factors " ? Not all we may hope to know some day, but a 

 great deal already, and there can be no doubt that they exist, 

 and that in biparental reproduction they follow certain well- 

 ascertained laws with the utmost exactness. By this I mean 

 that the phenomena to which they give rise follow in uniform 

 and invariable sequences. 



The evidence rests upon what is generally known as 

 " Mendelism," that is, the experimental study of breeding. It 

 is also called " Genetics." Being strictly experimental, this 

 evidence is of the highest scientific value. 



We have to show that there are " factors " in the germ- 

 plasm. I will take as an example of experiments demonstrat- 

 ing their presence the American fruit-fly, Drosophila ampelo- 

 phila, recently the subject of much study by Professor T. H. 

 Morgan and his collaborators. It is a small fly with red 

 eyes, a greenish - grey thorax, the abdomen banded yellow 

 and black, the single pair of wings grey, rounded at their 

 extremities and projecting considerably beyond the abdomen. 



In some individuals the wings are vestigial, reduced to 

 little fin-like projections from the mesothorax. If a normal 

 fly is crossed with a vestigial- winged fly, the offspring in Fl 

 (the first filial generation) is normal, with long wings. When 

 two of these offspring are interbred, their offspring (F2) 

 appear in the proportion of three long-winged to one vestigial. 



This is a definite numerical result and one cannot get 

 numerical results unless there are units in question, which 

 units behave independently. We may therefore assume the 

 presence of units and construct the following scheme : 



Let O be a unit for " vestigial " and a unit for " long." 



When the <? (male) and ? (female) cells of " vestigial " 

 and "long" are united in fertilisation we get ([) The 

 individuals reared in Fl from this combination are all 

 grey long, for , whenever present, always asserts itself, is 



