PRESIDENTIAL ADDRESS. 35 



for instance, that the simplification or even absence of respiratory 

 organs seen in Pauropus, in the Thysanura, and in other small 

 Tracheata, may be a secondary character, acquired through reduction 

 of size. 



An interesting illustration of the law discussed above is afforded by 

 the brains of mammals ; it has been noticed by many anatomists that 

 the extent of convolution, or folding of the surface of the cerebral 

 hemispheres in mammals, is related not to the degree of intelligence 

 of the animal, but to its actual size, a beaver having an almost smooth 

 brain and a cow a highly complicated one. Jelgersma, and inde- 

 pendently of him, Professor Fitzgerald, 1 have explained this as due to 

 the necessity of preserving the due proportion between the outer 

 layer of grey matter or cortex, which is approximately uniform in 

 thickness, and the central mass of white matter. But for the foldings 

 of the surface the proportion of white matter to grey matter would 

 be far higher in a large than in a small brain. 



It must not be forgotten, on the other hand, that many zoologists 

 hold the view, in favour of which the evidence is steadily increasing, 

 that the primitive or ancestral members of each group were of small 

 size. Thus Furbringer remarks with regard to birds, that on the whole 

 small birds show more primitive and simpler conditions of structure 

 than the larger members of the same group. He expresses the 

 opinion that the first birds were probably smaller than Archseopteryx, 

 and notes that reptiles and mammals also show in their earlier and 

 smaller types more primitive features than do their larger descendants. 

 Finally, Furbringer concludes that ' it is therefore the study of the 

 smallar members within given groups of animals which promises the 

 best results as to their phylogeny.' 



Again, one of the most striking points with regard to the pedigree 

 of the horse, as agreed on by palaeontologists, is the progressive 

 reduction in size which we meet with as we pass backwards in time 

 from stage to stage. The Pliocene Hipparion was smaller than the 

 existing horse, in fact about the size of a donkey ; the Miocene 

 Mesohippus about equalled a sheep ; while Eohippus, from the Lower 

 Eocene deposits, was no larger than a fox. Not only is there good 

 reason for holding that, as a rule, larger animals are descended from 

 1 Cf. Nature, June 5, 1890, p. 125. 



