324 AN INTRODUCTION TO MODERN GENETICS 



for which fundamental importance has been claimed. Lyssenko first 

 studied the technical device of vernaHzation, which enables one to alter 

 the date of ripening of a crop by suitable treatment (with heat, moisture, 

 etc.) of the seeds. This led him to the theory that the vegetative period 

 in the growth of a plant consists of a series of phases, during each of 

 which the plant demands certain particular external conditions. In 

 consequence, one variety may, in a certain region, be late because the 

 demands of an early phase are slow in being fulfilled, while another 

 variety may also be late but because of a slowing down of a later phase; 

 a suitable hybrid between the two complementary types may then be 

 earlier than either. It is probably too soon for a considered judgment 

 to be given of the value of this method of approach, but a priori it 

 seems likely that an analysis of the developmental effects of the genes 

 will prove valuable. Unfortunately, Lyssenko combines this apparently 

 sound theory with the wildest hypotheses, such as the denial of segre- 

 gation or of the existence of genes, and the rejection, on philosophical 

 grounds, of the whole edifice of genetics as developed since the 

 rediscovery of Mendel's work. 



Breeding for disease resistance has not been practised with any great 

 success in animals, though it is known that hereditary differences in 

 susceptibility occur.^ Thus native African cattle are often markedly 

 more resistant to local tick-borne diseases than are imported European 

 cattle, and Brahman cattle seem to be resistant to Texas fever; hybrids 

 with European cattle have been bred, but no definite breeds have been 

 started.^ Resistance, however, often seems to be polyfactorial and to 

 depend largely on environmental factors which are difficult to control, 

 and in practice selection would not be easy by any method less drastic 

 than allowing the disease to kill off all the animals it could. 



Even in animals, however, the search for favourable variations has 

 led outside the confines of the species. Not many of the species hybrids 

 which have been made have yet proved their worth in practice, except 

 the mule, which is an old-established type for certain purposes. The 

 cattalo, a cross between the cow and the American bison, may prove 

 valuable in the inhospitable plains of Texas; the Fi males are infertile, 

 but the females could be back-crossed and fertile hybrids were obtained 

 in later generations. 



^ Cf. Gowen 1937, Crew and Roberts 1933, Mohr 1934. ^ Cf. Rice 1934. 



