122 THE CANADIAN ENTOMOLOGIST" 



side, with results of extraordinary interest, not only to entomologists, but to 

 all biologists. In the earlier days Weismann and Merrifield, in later times 

 such men as Standfuss of Zurich and Morgan of New York, have produced 

 works of such value as to arrest the attention of all naturalists. In particular, 

 Morgan's intensive study of the vinegar-fly, Drosophila, has given us a knowl- 

 edge of the facts of variation and heredity which the most optimistic would 

 have declared impossible a few years ago. Thus all doubt as to the value of 

 minute and detailed investigation of single genera and species has been dis- 

 pelled, except in the minds of those who take no interest in the biological 

 problems of the day. Indeed, it must be said that any able student who will 

 study a single small group or species from all points of view, will be sure to get 

 results of value and importance, whereas as a collector of miscellanea he may go 

 through life without miaking any significant contribution to science. Thus the new 

 outlook and the new methods open up a great new field for amateurs, who 

 may readily make themselves more familiar than any one else with a special 

 small field of research, knowing at the same time that their discoveries will have 

 scm.e bearing on the whcjle structure of biological science. 



We approach the subject of variation to-day with many advantages not 

 enjoyed by our predecessors. Owing to the rediscovery of Mendel's work, and 

 the great advances in our knowledge of cytology and of the processes of heredity, 

 we are able to interpret what we find with better success. We no longer content 

 ourselves with describing, in objective terms, the phases of variation found, but 

 undertake to classify them according to their true dynamic significance. 



Variation may arise from different causes, as follows: 



1. Original variation, due to some change in the character of the germ plasm 

 itself. Theoretically, this may come about either through (a) the addition 

 of something, or (b) the subtraction of something, or (c) the redistribution 

 of what was already there, following the phenomena already well known 

 to students of organic chemistry. Tower, of Chicago, appears to have pro- 

 duced variations of this sort in potato-beetles (Leptinotarsa), but it is 

 possible to interpret them as the result of selective destruction of elements 

 (genes) in the germ plasm, which is, at least theoretically, a different 

 matter from altering the genes (factors determining characters) them- 

 selves. The sudden appearance of red on the rays of a sunflower in 

 Colorado can be interpreted as due to a doubling-up or duplication of a 

 gene for red which is undoubtedly present in the normal wild plant. 



Furthermore, when a variation occurs in a gamete (unfertilized 

 germ cell) which is recessive to the normal, — that is, fails to produce 

 any effect when united with a normal gamete, — it may be an indefinite 

 time, possibly a thousand years, before there will be any visible result. 

 A visible result will only appear when two individuals, each carrying the 

 modified character, chance to mate. Thus when we witness what ap- 

 pears to be an entirely new "break," we may be observing the conse- 

 quences of a chemical change which occurred long ago, the causes of 

 which, whatever they were, have long ceased to operate. 



The mcst important evidence has been obtained In- Professor 

 Morgan and his associates in their studies of Drosophila. In numerous 

 cases new variations have arisen under circumstances which seem to 



