106 PEOF. ERNEST W. MACBRIDE, M.A., E.R.S., ON 



is easy to point to the fact that the white rabbit, if crossed with 

 the wild rabbit, will give rise to progeny which will behave in a 

 Mendelian manner as regards colour, but the difference in size 

 and weight between the domesticated and wild varieties is not 

 thus got over. Of course, we may, if we will, extend the Men- 

 delian rules to cover differences of more or less, and this has 

 actually been done by some Mendelians. We may say either 

 that a mutation may cause only a slight increase or decrease in 

 some organ, but if we do this we are only repeating in pompous 

 phrase Darwin's statement that differences in size are sometimes 

 inherited — or we may suppose that different elementary species 

 are distinguishable from one another by the presence of factors 

 which cause slight differences in the proportions of certain 

 organs, so that by their crossing all intermediate grades can be 

 accounted for. The difficulty about meeting such a facile pre- 

 supposition as this is to devise means to bring it to a crucial 

 test. If we select bigger individuals from a species and by 

 mating them raise bigger offspring, and claim that this proves 

 an inheritability of differences of " more or less," the Mendelian 

 answers that in this case the difference in degree was due to 

 a mutation because it bred true, and thus we find ourselves 

 reasoning in a circle. 



If, then, the blind acceptance of the idea that the principles 

 of Mendel are the final word in the science of heredity leads to 

 the conclusion that the qualities or factors of the germ cells are 

 as unalterable as the chemical elements, let us put this theory 

 to the test of asking whether it explains the known facts of life. 

 In his Origin of Species Darwin emphasized the fact that the 

 record of past life on the earth is exceedingly defective, and that 

 all we have of it are bits and scraps. Broadly speaking, that 

 statement still holds s;ood, but since Darwin's time a few exces- 

 sively lucky finds have been made. We seem to have chanced 

 several times upon the actual locality where a type of animal 

 was evolved. In the Western States of North America there 

 once existed great inland lakes. These lakes, in due course, 

 became filled up with beds of mud and sand, brought down by the 

 rivers which flowed into them. As the lake became shallower 

 these deposits formed swampy meadows at its edges, and when 

 the animals that lived in the neighbourhood came down to 

 drink they were often bogged in these swamps and drowned. 

 Since these lakes existed for millions of years, we have embedded 

 in them a fair sample of the quadrupeds which inhabited the 

 neighbourhood, and in going from the earlier to the later of these 

 beds we notice changes in these animals, for instance we behold 



