THE RECAPITULATION THEORY 341 



increased complexity of structure ; the determining 

 consideration 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 proportions, 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 interchanges, 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 comparison with allied forms 

 suggests very strongly that there has been in its 



