INHERITANCE 183 



the Abbot Mendel. In their original form Mendel's laws 

 were based on the study of plant types, but they were at an 

 early stage extended to animals by Bateson (1902), whose 

 brilhant critique of the speculations of nineteenth- century 

 naturahsts in his " Materials for the Study of Variation " 

 (1895) had done so much to prepare the way for the 

 development of genetic physiology. 



In Mendel's original experiments inheritance was studied 

 in the common pea, which possesses a number of true-breeding 

 strains distinguished by well-defined characteristics such as 

 colour (yellow or green) of the seed coat, or size of shoot 

 (tall or dwarf). Mendel's method, which is the basis of all 

 truly quantitative treatment of inheritance on experimental 

 lines, differed from that of his predecessors in three particulars : 

 he confined his attention exclusively to the transmission of 

 single well-defined characteristics ; he recorded separately 

 the progeny of the individuals employed ; and he used in 

 his crosses only individuals from stocks proved to breed true 

 for such characteristics. Experiments not safeguarded by 

 this precaution have no value for purposes of scientific reason- 

 ing * in relation to our present problem, namely, the extent to 

 w^hich the sperm and egg respectively contribute in maintain- 

 ing the continuity of resemblance between parent and offspring. 



As an introduction to the problem let us consider the 

 results of mating an individual from a pure v/ild stock of the 

 fruit-fly Drosophila, in which the wings extend beyond the 

 tip of the abdomen, with an individual from a pure stock of 

 the well-established mutant (sport) in vvhich the v/ings are 

 vestigial. We shall attempt to build up the argument at each 

 stage in the experiment. The offspring of the first generation, 

 commonly referred to as the F.i (first fiHal), are all of the normal 

 (long- winged) type. Bodily they are indistinguishable from 

 the long-v/inged parent of the cross ; but when mated among 

 themselves they behave in a different manner, in that a definite 

 proportion of their progeny have vestigial wings. The long- 



* The pure line experiments of Johannsen are not so much to be re- 

 garded as having constructive significance, but rather as a means of clearing 

 up confused habits of thought resulting from the neglect of this precaution 

 in earlier work. — Author. 



