GENERAL SURVEY OF THE LIFE OF MAN. 149 



which it has been the purpose of the preceding section to point out and illus- 

 trate, to the history of Human Life, considered under its most general aspect. 

 The germ of the Human organism, derived from the vital operations of its 

 parents, must be considered as possessing a property or capacity, whereby, when 

 it is placed in the requisite material conditions, and subjected to a certain dynami- 

 cal agency, it evolves itself into the complete fabric, which is subsequently main- 

 tained by the continuance of the same agencies. And this property is so far 

 peculiar, that the germ of Man can never be evolved into any other form than 

 the Human, although it may attain this but very imperfectly. It is, however, 

 a purely passive capacity ; and the germ must be acted on by a force external to 

 it, before it can advance a single step in the developmental process. This force 

 is Heat, which, being supplied by the parental organism, is converted by the 

 instrumentality of the germ into the Vital force, whereby it appropriates the 

 nutrient materials supplied to it, and converts these into living tissue. 1 These 

 nutrient materials, prepared by the parent, are stored up in the ovum in suffi- 

 cient quantity to serve for the early development of the embryo, until it can 

 obtain them from other sources ; but while their quantity is adequate, in the 

 Oviparous animal, to serve for the evolution of its fabric into such a degree of 

 completeness as enables it to ingest and appropriate its further supplies for itself, 

 it suffices in the Mammal for little else than to enable the germ to evolve an 

 apparatus whereby it may receive a continued supply more directly imparted to 

 it from the bloodvessels of the mother. The first step in the process of evolu- 

 tion consists in mere growth, that is, in the multiplication of cells by duplicative 

 subdivision, without any departure from the primitive type ; but gradually we 

 see indications of development, in the multiplication of cells in particular direc- 

 tions, whereby the foundation is laid of the principal organs of the fabric, and 

 in the metamorphoses of certain of them into the tissues which are characteristic 

 of the perfect organism ; and it is in these two particulars alone, that the later 

 stages of embryonic life essentially differ from the earlier. During all this time, 

 Heat is being continually supplied by the parent and appropriated by the em- 

 bryo; which, at its period of maturity, exhibits the result of the continued 

 operation of the organizing force, and of its action, through the instrumentality 

 of the germ (in the first place) and (subsequently) of the living fabric that has 

 had its origin in it, upon the nutrient materials with which it has been sup- 

 plied. Up to this time, there has been very little expenditure of Vital force in 

 anything else than the formation of tissue ; for the life of the embryo is one 

 rather of organic or vegetative, than of animal activity ; the action* of the heart, 

 and the occasional reflex movements of the limbs, being its only manifestations 

 of nervo-muscular power. And thus it seems to be, that the formative capacity 

 is greater during embryonic life than at any subsequent period, and greater in 

 its earlier than in its later stages ; so that we have not only evidence of an 

 extraordinary power of regenerating parts which have been lost by disease or 

 accident, as shown in attempts at the reproduction of entire limbs after their 

 " spontaneous amputation;" but there is also not unfrequently an absolute 

 excess of productive power, as shown in the development of supernumerary 

 organs, which may even proceed to the extent of the complete duplication of the 

 entire body, by the early subdivision of the embryonic structure into two inde- 

 pendent halves. (See CHAP. xix. SECT. 4.) 



1 This is seen obviously enough in the incubation of Birds, in which the contact of the 

 surface of the body imparts that heat to the germ which it derives in Mammals from the 

 textures wherein it is imbedded. It is curious to observe that in several cold-blooded 

 animals, there is a special provision for generating heat, when the developmental processes 

 are being actively carried on ; as in the maturation of the pupae of Bees, and the evolution 

 of the embryo within the egg in certain viviparous Reptiles. See "Prin. of Phys., Gen. 

 and Comp.," 623 and 725, Am. Ed. 



