342 THE RECAPITULATION THEORY 



pedigree an actual reduction of size, which has led 

 to the degeneration of the respiratory organs. 



This is an important conclusion : it is a well- 

 known fact that the smaller members of a group 

 are as a rule more simply organised than the 

 larger members, especially with regard to their 

 respiratory and circulatory systems ; but if we are 

 right in concluding that reduction in size may be 

 an actual cause of simplification or degeneration in 

 structure, then we must be on our guard against 

 assuming hastily that these smaller and simpler 

 animals are necessarily primitive in regard to the 

 groups to which they belong. It is possible 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,* 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. 

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



