492 Mr. J. D. Dana on Species. 



3. Variations of species. 



But there are variations in species, and this is our next topic. 

 The principles ah-eady considered teach, as we beheve, that each 

 species has its specific vakie as a unit, which is essentially per- 

 manent or indestructible by any natural source of change ; and 

 we have, therefore, to admit in the outset, if these principles are 

 true, that variations have their limits, and cannot extend to the 

 obliteration of the fundamental characteristics of a species. 



To understand these variations, we may again appeal to 

 general truths. 



Variation is a characteristic of all things finite, and is involved 

 in the very conditions of existence. No substance or body can 

 be wholly independent of every or any other body in the uni- 

 verse. The most comprehensive and influential law in nature, the 

 most fundamental in all change, composition, or decomposition, 

 growth or decay, is the law of mutual sympathj^, or tendency to 

 equilibrium in force through universal action and reaction. 



The planets have their orbits modified by other bodies in space 

 through their changing relations to those bodies. A substance, 

 as oxygen or iron, varies in temperature and state of expansion 

 from the presence of a body of different temperature; in 

 chemical tendencies from the presence of a luminous body like 

 the sun ; in magnetic or electrical attraction from surrounding 

 magnetic or electi-ical influences. There is thus unceasing flow 

 and unceasing change through the universe. All the natural 

 fox'ces are closely related as if a common family or group, and 

 are in constant mutual interplay. 



The degree or kind of variation has its specific law for each 

 element ; and in this law the specific nature of the element is in 

 a degree expressed. There is to each body or species the nor- 

 mal or fundamental force in which its very nature consists ; and, 

 in addition, the relations of this force to other bodies, or kinds, 

 amounts or conditions of force, upon which its variations depend. 

 One great end of inorganic science is to study out the law of 

 variables for each element or species. For this law is as much 

 a part of an idea of the species, as the fundamental potentiahty; 

 indeed the one is a measure of the other. 



So again, a species in the organic kingdoms is subject to varia- 

 tions, and upon the same principle. Its very development de- 

 pends on the appropriation of material around it, and on attend- 

 ing physical forces or conditions, all of which are variable through 

 the whole of its history. Every chemical or molecular law in 

 the universe is concerned in the growth, — the laws of heat, 

 light, electricity, cohesion, &c. ; and the progress of the de- 

 veloping germ, whatever its primal potentiahty, is unavoidably 

 subject to variations, from the diversified influences to which it 



