1893. 



EVOLUTION OF THE MAMMALIA. 99 



with the great extension of grassy plains. It may, therefore, be 

 correlated with this extension of grass lands ; and we may infer that 

 the complex grinding teeth arose as a consequence of a change of 

 food-supply presented to the herbivorous mammals from a diet con- 

 sisting of soft plants and leaves to a diet consisting mainly of 

 siliceous grasses. " Now, what is the probability," asks Dr. Scott, 

 "that such a series of changes in horses, rhinoceroses, pigs, rumi- 

 nants, elephants, and other families, should be due primarily to the 

 mingling of different hereditary tendencies, especially when it is 

 remembered that none of the ancestors of these groups possessed 

 any such teeth ? Or can it be reasonably contended that such 

 parallel variations are due to the direct action of the climatic or 

 other environment upon the germ-plasm ? " 



According to Dr. Weismann, it must be remembered, variations 

 arise in the main from the combinations of slightly differing develop- 

 mental tendencies in the elements derived from the parents which 

 unite to form the offspring. Nothing acquired during the life of 

 either parent, or both parents, can affect the germinal or fertilising 

 element. It is, therefore, a matter of chance whether any new varia- 

 tion shall be well-adapted to somewhat new conditions of life, or shall 

 be ill-adapted to such new conditions ; but the well-adapted survive, 

 while the ill-adapted succumb. How is it, asks Dr. Scott, that we 

 find only favourable adaptations ? Where are the unfavourable 

 adaptations that succumbed ? And how comes it that in parallel 

 series of animals similar favourable adaptations so constantly occur, 

 if these similar favourable adaptations be not due to the direct 

 influence of similar environing circumstances ? 



In a presidential address to the Bristol Naturalists' Society, I 

 briefly considered the bearing and value of such palaeontological 

 evidence in the vexed question of " The Nature and Origin of 

 Variations," and I ventured to express a doubt as to whether the 

 evidence could be regarded as conclusive as to " use-inheritance," or 

 the transmission to the offspring of modifications of structure acquired 

 by the parents during their individual life-time. In criticising the 

 position occupied by Dr. Scott and those whose views are in 

 accordance with his, I thought that some stress might be laid on 

 the imperfection of the geological record, and that the number of 

 individuals in our palaeontological collections was not sufficient to 

 constitute a truly representative sample. Furthermore, I suggested 

 that, on the hypothesis of selection, the individuals possessed of 

 unfavourable, unadaptive modifications must have been weeded out 

 in the early stages of life. In reply to such criticisms, Dr. Scott 

 says " that the objection drawn from the imperfection of the geo- 

 logical record, and from the small number of individuals contained 

 in the museums, can be allowed little weight. So far as several of 

 the mammalian phyla are concerned, the number of missing links 

 must be very small, the wonderful series of American freshwater 



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