FUTURE OF THE SCIENCE OF BREEDING 163 



this is confirmed by the fact that the new types can at once be 

 fixed and made into true-breeding strains. 



The illustration just given is of purpose a simple one, 

 but it is clear that if the original parents differ in a larger 

 number of factors the diversity of the second generation will 

 be correspondingly greater. The recognition of this principle 

 of the orderly assortment of factors among the gametes has 

 rendered possible that detailed analysis of plants and animals 

 which has made such rapid strides during the past few years. 

 Analysis of this kind aims at the determination of every factor 

 in a species by which one individual may differ from another. 

 Mendel himself determined seven such factors in the pea. 

 The number has now grown to thirty-five and includes such 

 characters as flower-colour, seed-shape, height, pod-structure, 

 foliage-form and flowering period. Many factors have also 

 been definitely determined in the cereals and in other plants, 

 though in no case is the analysis approaching completion. 

 Since upon it depends the breeder's estimate of the possibilities 

 of making new and improved strains, as well as the facility with 

 which he is able to attain his end, it is natural that attention 

 in many quarters should be more directly focussed upon 

 plants of economic value. Owing to the heavier expenses 

 connected with their breeding, animals have not been analysed 

 to the same extent. There is, however, one valuable exception, 

 the little pomace fly (Drosophila), in which Professor Morgan 

 and his collaborators claim to have determined over one 

 hundred different factors. To this small and important 

 creature we shall have to return later. 



Now all this work is merely an elaboration of Mendel's 

 original communication. The method was laid down by him, 

 and the theoretical conceptions upon which the method was 

 based were clearly stated in his masterly essay. Though the 

 work is of to-day we are dealing with the scientific past. 

 Nevertheless the geneticist is not yet brought to a standstill. 

 There is for him a scientific present full of novel conceptions 

 which are being put vigorously to the test. A keen spirit of 

 controversy animates the genetic world always a healthy 

 sign. I may perhaps dwell briefly upon a few of these 



