ATOM. 461 



contains about two million molecules of organic matter. At least half of every 

 living organism consists of water, so that the smallest living being visible 

 under the microscope does not contain more than about a million organic 

 molecules. Some exceedingly simple organism may be supposed built up of 

 not more than a million similar molecules. It is impossible, however, to conceive 

 so small a number sufficient to form a being furnished with a whole system 

 of specialised organs. 



Thus molecular science sets us face to face with physiological theories. 

 It forbids the physiologist from imagining that structural details of infinitely 

 small dimensions can furnish an explanation of the infinite variety which exists 

 in the properties and functions of the most minute organisms. 



A microscopic germ is, we know, capable of development into a highly 

 organised animal. Another germ, equally microscopic, becomes, when developed, 

 an animal of a totally different kind. Do all the differences, infinite in number, 

 which distinguish the one animal from the other, arise each from some dif- 

 ference in the structure of the respective germs ? Even if we admit this as 

 possible, we shall be called upon by the advocates of Pangenesis to admit 

 still greater marvels. For the microscopic germ, according to this theory, is 

 no mere individual, but a representative body, containing members collected 

 from every rank of the long-drawn ramification of the ancestral tree, the number 

 of these members being amply sufficient not only to furnish the hereditary- 

 characteristics of every organ of the body and every habit of the animal from 

 birth to death, but also to afford a stock of latent gemmules to be passed 

 on in an inactive state from germ to germ, till at last the ancestral peculiarity 

 which it represents is revived in some remote descendant. 



Some of the exponents of this theory of heredity have attempted to elude 

 the difficulty of placing a whole world of wonders within a body so small 

 and so devoid of visible structure as a germ, by using the phrase structureless 

 germs*. Now, one material system can differ from another only in the con- 

 figuration and motion which it has at a given instant. To explain differences 

 of function and development of a germ without assuming differences of structure 

 is, therefore, to admit that the properties of a germ are not those of a purely 



material system. 



The evidence as to the nature and motion of molecules, with which we 

 have hitherto been occupied, has been derived from experiments upon gaseous 

 * See F. Galton, "On Blood Relationship," Proc. Roy. Soc., June 13, 1872. 



