224 AGE AND AREA [pt. ii 



has been prepared by Gates in his new book on Mutations and 

 Evolution. 



In the fruit-fl}^ Drosophila over two hundred instances have 

 been studied by Morgan and his co-workers, and the evening 

 primroses, or Oenotheras, have afforded some dozens, many of 

 which differ more Avidely from their parent form than recog- 

 nised wild species of this polymorphic genus do among them- 

 selves. On the other hand, no observations have been adduced 

 of new forms originating experimentally from fluctuating vari- 

 ations. 



The experimental work has not, however, chosen for its scope 

 the proof of the reality of mutations, but has preferred other 

 lines of research. In the studies of Morgan the distribution of 

 the genes along the chromosomes, as predicted from the prin- 

 ciple of Pangenesis, has been the main aim. With Oenothera the 

 prominent question was the search for a method of studying 

 the internal and external causes, Avhich induce mutations to 

 occur repeatedly. A thorough knowledge of these causes must, 

 in the end, enable us to produce artificially distinct changes, 

 determined beforehand. In other words, it must afford the means 

 of evolving arbitrarily new useful varieties of chosen qualities, 

 in agricultural and in horticultural plants. 



In systematic studies it is now generally recognised that the 

 characters used in the diagnostic distinction of related species 

 are not such as would be expected on the ground of Darwin's 

 selection theory. As a rule they relate to qualities, which cannot 

 be explained on the assumjjtion of an origin by the accumulation 

 of infinitesimal steps on the basis of their usefulness for the 

 species. They are not observed to increase the chance of success 

 in the struggle for life. Most forms would thri^•e as well without 

 their aid. This is especially the case with morphological charac- 

 ters, whereas adaptation to such environmental conditions as 

 moisture or drjaiess, shadow or open field, physical and chemical 

 constitution of the soil, etc., might far more easily be imagined 

 to evolve slowly. But even here direct jjroofs are wanting. 



It is a curious fact that most of the striking instances of 

 beautiful adaptation to special forms of life are characters of 

 genera and subgenera, or even of whole families, but not of 

 single species. Climbing plants and tendrils, insectivorous plants, 

 desert types of Cactus, Euphorbia, and so many others, sub- 

 merged water plants, and numerous other instances could be 

 adduced. Since we do not know when and where and under 



