1912. Irish Societies. 8i 



and the Lamarckiaii factor of use-inhoritancc still goes on, not always in the 

 most strictly scientific spirit. Natural selection explains readily adaptive 

 characters that arc 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 popiili has a green tail -horn and the closely similar larva of 

 S. ocellatiis 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 they are unable to affect, ruling use -inheritance out of court. 

 On the other hand we have the belief, still held by man}^ that the body 

 of the individual, affected by exercise or environment, can influence the 

 germ -cells that it encloses. In support of this view there were put forward 

 in former years many unconvincing experiments of the nature of mutila- 

 tion and much ingenious speculation. Recently we have had the suggestive 

 and valuable breeding experiments on salamanders and toads carried out 

 by Dr. P. Kammerer, of Vienna, He found, for example, that Spotted 

 Salamanders subject to cold and drought produced young approaching 

 those of the Black Salamander, and that three years later the offspring of 

 these, breeding in conditions normal to their species, produced young still 

 resembling somewhat those of the black kind. Such facts seem to tell 

 strongly in favour of use-inheritance, yet it must be admitted that the 

 germ-cells of the salamanders may be directly affected by the surroundings, 

 and that the phenomena in such obviously plastic organisms may be due 

 to " ancestral conditions latent in the germ -cells, and called forth by 

 appropriate stimuli." However probable may seem the action of use 

 inheritance, we must still admit it " not proven." Though Darwin, in his 

 late years, attached much importance to slight " continuous " variations 

 in the production of new species, discontinuous variations may be of great 

 v^alue in the evolutionary process, and the study 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 in the evolution theory 

 during the past ten years have undoubtedly been along the lines of Men- 

 delian research. The simpler results at least of the Mendelian 

 breeding experiments are now generally known among naturalists. 

 With the light thrown on reproductive operations by these results — 

 harmonising as they most wonderfully do with observed facts as to the 

 reducing nuclear divisions in the maturation of the germ cells — the practical 

 breeder of animals and plants can often combine characters so as to pro- 

 duce within limits certain desired results, while the student of evolutionary 

 theory perceives how differentiated characters can be segregated and fixed 

 apart altogether from their utility or from the action of use-inheritance. 

 The most recent advances in Mendelian work make it at least highly prob- 

 able that in many animals sex-differentiation is itself a Mendelian factor. 



