216 L. Agassiz on the Natural Relations between 



whole, and taking into consideration the proportional num- 

 ber of their genera and species, of larger size and greater 

 weight than the fluviatile. We have nowhere such gigantic, 

 bulky, and heavy fresh-water Bivalves, as are many of the 

 marine shells ; and we need only compare the large Chamas 

 or Tridacnas and Hippopus, the gigantic Pinna, even with 

 the largest of Anadonts ; and again, the numerous species of 

 Cyclas, &c., with the smaller marine Bivalves, among which 

 we find but few species of so minute types. Again, among 

 Gasteropoda, how much larger are most of the Univalve 

 marine shells, such as Dolium, Strombus, Voluta, and others, 

 than even the largest fresh-water Ampullarise, and the whole 

 lot of fresh-water and terrestrial Pulmonata, among which 

 latter we have absolutely the smallest of all Mollusca in the 

 innumerable varieties of Pupa and other genera. We reckon, 

 in this type of Gasteropoda, the minute species by hundreds, 

 while there are exceedingly few of really small size among 

 the marine ones ; and the greater number are even universally 

 above the medium size of the larger fluviatile and terrestrial 

 types. 



Among Articulata the same rule obtains; and here we 

 may compare classes with classes, even in their difi^erent stages 

 of growth. Are not the Worms, taken as a whole, larger 

 animals than the Caterpillars? Do we not find among 

 marine Worms by far the largest types ? We need only 

 remember the gigantic Eunice, or even the parasitic Tape- 

 worms, to be satisfied of the fact. Are not the Crustacea, 

 as a class, composed of types exceeding far the largest in- 

 sects, even with their wings spread ? Are not the marine 

 Lobsters many times larger than the fresh-water Craw -fishes? 

 A minute investigation of the details of this numerous class 

 might lead to very interesting comparisons, which, however, 

 would be out of the way in this general sketch. 



I shall mention only a few facts to shew that these com- 

 parisons might even be traced between the different stages 

 of growth of these animals. It must be, for instance, a 

 matter of surprise to see that the body of so many insects is 

 smaller in their perfect state of development than as a pupa ; 

 and that again, this is smaller than that of the larva, though 



