84 SECTIONAL ADDRESSES 



type. But if vestigials and normals are kept together without food and water, 

 the vestigials survive longer. Thus, in environments which occasionally 

 become very unfavourable, the vestigial type might even oust the normal. 



In dandelions, Sukatschew has carried out elaborate experiments on 

 a number of pure lines. Altering the density of total numbers of plants 

 per plot may completely alter both the survival of the seedlings and the 

 fertility of the survivors, so that a pure line which is inferior in one set 

 of conditions will oust the rest in other conditions. This conclusion is 

 entirely in accord with the work of Stapledon and others showing the effect 

 of varying intensity of grazing on the survival and reproduction of 

 different species and strains of pasture plants. 



A striking case of rather a different nature concerns a variety, probably 

 due to mutation, observed in tobacco. The new variety failed to flower 

 until the ratio of light to darkness was altered to correspond with what 

 would prevail in a semi-tropical summer, when it became a better per- 

 former than the type. Any competition between mutant and type would 

 thus be decided according to latitude. 



I ought also to mention the case, described by Harrison, of the light 

 and dark varieties of the moth Oporinia autumnata. The relative abun- 

 dance of these in a dark pinewood and an adjacent light birchwood is 

 quite different, and so, but inversely, is the intensity of selection, as re- 

 vealed by the number of wings left by birds. The result is that in the 

 dark environment the dark variety is sixteen times the commoner, in the 

 light environment six times the rarer. 



Thus, whatever other processes may possibly be at work, it is clear 

 that selection is constantly operative. A difference in environment may 

 decide between two genes with sharply contrasting effects ; quantitative 

 differences in conditions may lead to a complete reversal of advantage 

 between varieties ; the gene-complex may be selected so as to protect 

 the species from the deleterious effects of mutations, or so as to minimise 

 the ill effects of an otherwise advantageous mutant. In these and other 

 ways natural selection proves itself to be a pervading, active agency. 



Having dealt briefly with the modus operandi of natural selection in a 

 Mendelian world, we must now discuss the processes of evolution and 

 the role which selection may play in them. Darwin himself happened 

 to confuse the issue by calling his greatest book the Origin of Species. 

 Evolution, however, must be dealt with under several rather distinct 

 heads. Of these one is the origin of species — or we had better say 

 the origin of minor systematic diversity. Another is the origin of 

 adaptations. A third is extinction. And a fourth, and in many ways the 

 most important, is the origin and maintenance of long-range evolutionary 

 trends. It is of course true that these all overlap and interlock. None 

 the less, the distinctions are real and important. 



The Origins of Species. 



First, then, we have the origin of species. It is logically obvious that 

 every existing species must have originated from some pre-existing 

 species, but equally clear on the basis of recent research that it may do so 

 in one of several quite different ways. A single species as a whole may 



