OF LIFE, AND ITS CONDITIONS. 29 



germ; which 1ms been supposed to derive from its parent not merely its 

 material substance, but a nisus formativus, bildung-strieb, or germ-force, in 

 virtue of which it builds itself up into the likeness of its parent, and main- 

 tains itself in that likeness until the force is exhausted, at the same time 

 imparting a fraction of it to each of its progeny. In this mode of viewing 

 the subject, all the organizing force required to build up an Oak or a Palm, 

 an Elephant or a Whale, must be concentrated in a minute particle only 

 discernible by microscopic aid ; and the aggregate of all the germ-forces 

 appertaining to the descendants, however numerous, of a common parent- 

 age, must have existed in their original progenitors. Thus, in the case of 

 the successive viviparous broods of Aphides, a germ-force capable of organ- 

 izing a mass of living structure, which would amount (it has been calcu- 

 lated 1 ) in the tenth brood to the bulk of five hundred millions of stout men, 

 must have been shut up in the single individual, weighing perhaps the 

 7Foo tn f a g r ai u . from which the first brood was evolved. And in like 

 manner, the germ-force which has organized the bodies of all the individual 

 men that have lived from Adam to the present day, must have been con- 

 centrated in the body of their common ancestor. A more complete reductio 

 ad absurdum can scarcely be brought against any hypothesis; and we may 

 consider it proved that, in some way or other, fresh organizing force is con- 

 stantly being supplied from without during the whole period of the exercise 

 of its activity. When we carefully look into the question, we fiud that 

 what the germ really supplies is not the force, but the directive agency; thus 

 rather resembling the control exercised by the superintendent builder who 

 is charged with the working-out the design of the architect, than the bodily 

 force of the workmen who labor under his guidance in the construction of 

 the fabric. The actual constructive force, as we learn from an extensive 

 survey of the phenomena of life, is supplied by Heat; the influence of which 

 upon the rate of growth and development, both Animal and Vegetable, is 

 so marked as to have universally attracted the attention of Physiologists; 

 who, however, have for the most part only recognized in it a vital stimulus 

 that calls forth the latent power of the germ, instead of looking upon it as 

 itself furnishing the power that does the work. It has been from the nar- 

 row limitation of the area over which Physiological research has been com- 

 monly prosecuted, that the intimacy of this relationship between Heat and 

 the Organizing force has not sooner become apparent. Whilst the vital 

 phenomena of Warm-blooded Animals, which possess within themselves the 

 means of maintaining a constant temperature, were made the sole, or at any 

 rate the chief objects of study, it was not likely that the inquirer would 

 recognize the full influence of external Heat in accelerating, or of Cold in 

 retarding, their functional activity. It is only when the survey is extended 

 to Cold-blooded Animals, and to Plants, that the immediate and direct rela- 

 tion between Heat and Vital Activity, as manifested in the rate of growth 

 and development, or of other changes peculiar to the living body, is unmis- 

 takably manifested. To some of those phenomena which afford the best 

 illustrations of the mode in which Heat acts upon the living organism, at- 

 tention will now be directed. 



8. The agency of Heat as the "efficient cause" or " motive power" to 

 which the phenomena of growth and development are to be referred, is 

 peculiarly well seen in the process of Germination. The Seed consists of an 

 embryo which has already advanced to a certain stage of development, and 

 of a store of nutriment laid up as the material for its further evolution; and 



1 See Prof. Hnxley on the "Agamic Reproduction of Aphis," in Linnsean Trans- 

 si tions, vol. xxii, p. 215. 



