34 PROFESSOR A. M. MAESHALL. 



In adult animals the relation between size and structure has been 

 very clearly pointed out by Herbert Spencer. Increased size involves 

 by itself increased complexity of structure ; the determining con- 

 sideration being that while the surface area of the body increases as 

 the squares of the linear dimensions, the mass of the body increases 

 as their cubes. 



If, for example, we imagine two animals of similar shape and pro- 

 portions, but of different size ; for the sake of simplicity, we may 

 suppose them to be spherical, and that the diameter of one is twice 

 that of the other ; then the larger one will have four times the extent 

 of surface of the smaller, but eight times its mass or bulk : and it is 

 quite possible that while the extent of surface, or skin, in the smaller 

 animal might suffice for the necessary respiratory and excretory inter- 

 changes, it would be altogether insufficient in the larger animal, in 

 which increased extent of surface must be provided by foldings of the 

 skin, as in the form of gills. 



To take an actual instance ; Limapontia is a minute nudibranchiate, 

 or sea-slug, about the sixth of an inch in length ; it has a smooth 

 body, totally devoid of respiratory processes, while forms allied to it, 

 but of larger size, have their extent of surface increased by branching 

 processes, which often take the form of specialised gills. 



This is a peculiarly instructive case, because Limapontia in its early 

 developmental stages possesses a large spirally-coiled shell, and shows 

 other evidence of descent from forms with specialised breathing 

 organs. We are certainly right in associating the absence of respiratory 

 organs in the adult with the small size of the animal ; and com- 

 parison with allied forms suggests very strongly that there has been 

 in its 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 de- 

 generation 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, 



