c-o [Pioc. B.N.P.C, 



are often common to large zoological groups. But can it explain 

 specific differences that have no apparent utility to their possessors ? 

 Can it be due to natural selection, for example, that the caterpillar 

 of Smerinthus populi has a green tail-horn and the closely similar 

 larva of 6". ocellatus a blue one ? And an enormous number, 

 probably the majority, of specific differential characters are of such 

 a nature. Yet there can be no doubt that the influence of natural 

 selection on the world of life is great and continuous. Can as 

 much be said for the use-inheritance ? On the one hand we have 

 that great Darwinian Professor Weismann, with his theory of the 

 " continuity of germ plasm " from one set of gametes to the next, 

 the bodies of successive generations regarded almost as bye- 

 products of the germ cells, which the)' are unable to affect, ruling 

 use-inheritance out of court. On the other hand we have the 

 belief, still held by many, that the body of the individual, affected 

 by exercise or environment, can influence the germ cells that it 

 encloses. The basal factor of evolution is heredity, with variation. 

 We may ask next what recent advances have been made in our 

 knowledge of variation, its nature and its causes. Darwin, at any 

 rate in his later years, was disposed to attach much importance to 

 comparatively slight "continuous " variations, and Francis Galton's 

 statistical studies originated the modern " biometric " school, of 

 whose methods the late Professor W. R. Weldon gave a brilliant 

 exposition in a popular lecture delivered at Belfast before the 

 British Association in 1902. The offspring of exceptional parents 

 appear, when large numbers are treated statistically, to regress 

 towards type. Divergences of measurable characters from the 

 normal tend to fall along a symmetrical curve on either side of the 

 mean. On the other hand, Professor Bateson, De Vries, and 

 others have laid stress on the importance of " discontinuous " 

 variations or " mutations." The fact of discontinuous variation 

 leads us directly to the work of Mendel, published in 1865, 

 neglected for more than thirty years, and rediscovered only at the 

 very end of the nineteenth century. The most marked advances 



