Mr. J. D. Dana on Zoophytes. 103 



numbers : and this is so distinct in some classes, that it becomes 

 an interesting study to trace out the sources of variations from 

 the typical number *. We see the boundless resources of nature's 

 Author displayed with greater force, the fewer the types from 

 which an infinite variety might proceed ; but not in any limiting 

 of the number of species constituting groups. 



Among the organs upon which the range of characters in the 

 animal kingdom depends, the nervous system takes necessarily 

 the precedence, for, as has been said with much propriety, this 

 system is itself the species; since upon its characters, in con- 

 nexion with the general laws of organic growth, depends in a 

 very great degree the nature of the individual. Next to this, come 

 those organs which are intimately connected with the sustaining 

 of life, primarily, those pertaining to respiration and circulation, 

 and secondarily, those adapted to the receiving and digesting of 

 food ; and next, or of parallel value with the last, the provision 

 for the continuation of the species. The means of locomotion 

 and the associated structure, constitute a characteristic intimately 

 connected with the causes just mentioned. Under the several 

 grand divisions to which we are led by the above considerations, 

 there are subordinate variations arising from the adaptation of life 

 to minor differences in the conditions of existence around us : — 

 such as minor differences of soil (if we may extend this word to 

 all those varieties of sites, afforded by the air, earth, water, vege- 

 table and animal structures, variously modified by temperature, 

 light and pressure) ; differences in the modes of taking prey or 

 food of whatever kind, and in some peculiarities of the organs of 

 digestion ; certain differences depending on the sexual relations, 

 and the means of preserving and developing the young, varying 

 with the modes of existence alluded to; modifications of the pro- 

 visions for self-preservation against enemies. These minor differ- 

 ences are exhibited in two ways : either particular organs retaining 

 the same functions, undergo modifications in form and structure ; 

 or with other modifications, they subserve the purposes of different 

 functions. When adaptations to different circumstances or pur- 



* Milne Edwards has well illustrated the fact, that seven is a normal 

 number in Crustacea, the cephalic, thoracic and abdominal parts each con- 

 sisting normally of this number of segments, and variations taking place by 

 a union of two or more segments, or by subdivisions ; and this same law 

 extends even to the joints of the legs and antenna?. The prevalence of 

 such a law through so large a class affords a sufficient ground for belief 

 that specific numbers have not been entirely disregarded in any branch of 

 nature, though the actual exhibition of them has been obscured in ways 

 not understood. We cannot disbelieve, therefore, that numerical relations 

 were involved in the plan of creation ; yet, while admitting them as regards 

 the nature of organic structures, we do not admit that the number of struc- 

 tures made on any particular type had reference to any similar ratio. 



