THE FACTORS OP OKGANIC EVOLUTION. 39 



height. Moreover, having previously supposed that he 

 understood the ascent of a balloon when he ascribed it to 

 relative lightness, he now sees that he did not truly under 

 stand it. For he did not recognize it as a result of that 

 upward pressure caused by the difference between the 

 weight of the mass formed by the gas in the balloon phis 

 the cylindrical column of air extending above it to the limit 

 of the atmosphere, and the weight of a similar cylindrical 

 column of air extending down to the under surface of the 

 balloon : this difference of weight causing an equivalent 

 upward pressure on the under surface. 



Why do I introduce these familiar truths so entirely irre 

 levant to my subject ? I do it to show, in the first place, 

 the contrast between a vague conception of a cause and a 

 distinct conception of it; or rather, the contrast between 

 that conception of a cause which results when it is simply 

 classed with some other or others which familiarity makes 

 us think we understand, and that conception of a cause 

 which results when it is represented in terms of definite 

 physical forces admitting of measurement. And I do it to 

 show, in the second place, that when we insist on resolving 

 a verbally-intelligible cause into its actual factors, we 

 get not only a clear solution of the problem before us, but 

 we find that the way is opened to solutions of sundry other 

 problems. While we rest satisfied with unanalyzed causes, 

 we may be sure both that we do not rightly comprehend the 

 production of the particular effects ascribed to them, and 

 that we overlook other effects which would be revealed 

 to us by contemplation of the causes as analyzed? Espe 

 cially must this be so where the causation is complex. 

 Hence we may infer that the phenomena presented by 

 the development of species, are not likely to be truly 

 conceived unless we keep in view the concrete agencies;) at 

 work. Let us look closely at the facts to be dealt with. 



The growth of a thing is effected by the joint operation 



