AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 83 



conditions of existence. No substance or body can be wholly independent of 

 every or any other body in the universe. The most comprehensive and influential 

 law in nature, most fundamental in all change, composition or decomposition, 

 growth or decay, is the law of mutual sympathy, or tendency to equilibrium in 

 force through universal action and reaction. 



The planets have their orbits modified by other bodies in space thi'ough their 

 changing relation to those bodies. A substance, as oxygen or iron, varies in tem- 

 perature and state of expansion from the presence of a body of different tempera- 

 ture ; in chemical tendencies from the presence of a luminous body like the sun ; 

 in magnetic or electrical attraction from surrounding magnetic or electrical 

 influences. There is thus unceasing flow and unceasing change through the 

 universe. All the natural forces are closely related as if a common faoiily 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 tlie element is in a degree expressed. Tliere is to 

 each body or species the normal or fundamental force in which its very nature 

 consists ; and in addition, the relation 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 funda- 

 mental potentiality ; indeed the one is a measure of the other. 



So again, a species in the organic kingdoms is subject to variations, and upon 

 the same principle. Its very development depends on the appropriation of 

 material around it, and on attending physical forces or conditions, all of which are 

 variable through the whole of its history. Every chemical or molecukr law in 

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

 cohesion, <Sic. ; and the progress of the developiog germ, whatever its primal 

 potentiality, is unavoidably subject to variation, from the diversified influences to 

 which it may be exposed. The new germ, moreover, takes peculiarities from the 

 parent, or from- the circumstances to which its ancestry had been exposed during 

 one or more preceding generations. 



There is then a fixed normal condition or value, and around it librations take 

 place. There is a central or intrinsic law which prevents a species being drawn 

 off to its destruction by any external agency, while subject to greater or less 

 variations under extrinsic forces. 



Liability to variation is hence part of the law of a species ; and we cannot be 

 said to comprehend in any case the complete idea of the type until the relations 

 to external forces are also known. The law of variables is as much an expression 

 of the fundamental equalities of the species in organic as in inorganic nature ; 

 and it should be the great aim of science to investigate it for every species. It is 

 a source of knowledge which will yet give us a deep insight into the fundamental 

 laws of life. Variations are not to be arranged under the head of accidents : for 

 there is nothing accidental in nature ; what we so call, are expressions really of 

 profound law, and often betray truth and law which we should otherwise never 

 suspect, 



This process of variation is the external revealing the internal, through their 

 sympathetic relations ; it is the law of universal nature reacting on the law of a 

 special nature, and compelling the latter to exhibit its qualities ; it is a centre of 



