92 



On the Classification of Animals 



14. Genetic— -Number of young or eggs. — As is well known, 

 tliere is a mark of grade in the number of eggs or young pro- 

 duced at a single period or in a given time — the number, other 

 things equal, being inversely as the rank or grade of the species. 



15. Thermotic. — Temperature required for emhryonic develop- 

 ment. — Another mark of grade is afforded by the temperature 

 required for egg-development : — for, in general, the higher the 

 temperature, the higher the grade. Thus, the eggs of Birds re- 

 quire heat above ordinary summer heat, while those of Reptiles 

 do not. The embryos of Mammals require still higher and more 

 uniformly continued heat until their maturity^ the Obtocoids alone 

 excepted, in which birth is premature. The eggs of some Hy- 

 menopterous Insects mature inside of the larves of other Insects, 

 where they are never exposed to a temperature of 32° Fahr. ; 

 while those of ordinary Lepidopters and many other species 

 mature in the summer heat, and may stand a temperature be- 

 low 0° Fahr. 



The necessity of a higher temperature indicates, ordinarily, 

 that the chemical processes in the vital economy are of a higher 

 or more delicate character, or those required for a higher grade of 

 cephalization. 



G. Geographical Distribution. 



16. Habitationcd. — (1.) Terrestrial species higher than Aquat ic. — 

 This law, announced by Agassiz, is also directly dependent on 

 the conditions determining the grade of cephalization. 



a. In the case of aquatic species, the ova, as well as the adult 

 animals, are bathed in a liquid that penetrates to the interior, and 

 dilutes, to some degree, the nutrient or developing fluids ; and, 

 under such circumstances, the grade of chemical or vital evolution 

 cannot be as high as in the atmosphere. The germ must there- 

 fore be one of an inferior kind. Aquatic animals are, in an im- 

 portant sense, diluted animals. 



6. Again, terrestrial species whose ova are hatched in water, or 

 whose young are aquatic, are for the same reason inferior, as a 

 general rule, to those w^hose ova are hatched on the land. 



Aquatic development or life is one of the most important marks 

 of low grade. Among embryological characteristics, it has often 

 a profounder value than prematurity. The inferior division of a 

 class, order, tribe, and even subordinate group, is often one con- 

 sisting either of aquatic species, or those that are semi-aquatic 

 (aquatic in habit though not strictly so in mode of life, or aquatic 

 in the young state when not in the adult). 



(2.) Living {<x)in imp)ure waters, or those abnormal in condition; 

 or (b) in deficient light, as in shaded places, or the ocearCs depths, 

 a mark of inferiority. — Muddy waters, or salt waters excessively 



