56 INTRODUCTION. 



lvalue in Physiological Science. Among the most important pf 

 ! these, is the complete metamorphosis which has been effected in 

 the ideas previously entertained regarding living action; such 

 having been essentially based on the Circulation of the blood, as 

 the only vital j1!ienomenon of which any direct cognizance could 

 be gained through the medium of the senses. For it gradually 

 came to be clearly perceived, that in the Animal as in the Plant, 

 each integral portion of the Organism possesses an independent Life 

 of its oivn, in virtue of which it performs a series of actions pe- 

 culiar to itself, provided that the conditions requisite for those 

 actions be supplied to it ; and that the Life of the body as a 

 whole (like a symphony performed by a full orchestra) consists 

 in the harmonious combination of its separate instrumental acts, 

 — the circulation of the blood, instead of making the tissues, sim- 

 ply affording the supply of prepared nutriment, at the expense 

 of which they evolve themselves from germs previously existing. 

 This general doctrine was first put prominently forwards by 

 Schwann, whose " Microscopical Researches into the Accordance 

 in the Structure and Growth of Animals and Plants," published 

 in 1839, marks the coinmencement of a new era in all that 

 department of Animal Physiology, which comprises the simply 

 vegetative life of the organized fabric. These researches, avow- 

 edly based upon the ideas advanced by Schleiden, were prose- 

 cuted in the same direction as his had been ; the object which 

 this admirable observer and Yihilosophic reasoner specially pro- 

 posed to himself, being the study of the development of the 

 Animal tissues. He found that although their evolution cannot 

 be watched while in actual progress, its history may be traced 

 lout by the comparison of the successive stages brought to light 

 Iby Microscopic research ; and in so far as this has been accom- 

 plished for each sepai-ate part of the organism, the structure and 

 actions of its several components, however diverse in their fully 

 developed condition, are found to resemble each other more and 

 more closely, the more nearly these parts are traced back to their 

 earliest appearance. Thus we arrive in our retrospective survey, 

 at a period in the early history of Man, at which the whole em- 

 bryonic mass is but a congeries of cells, all apparently similar and 

 equal to each other ; and going still further back, it is found 

 that all these have had their origin in the subdivision of a single 

 primordial cell, which is the first defined product of the generative 

 act. On this single cell, the Physiologist bases his idea of the 

 most elementary type of Organization ; whilst his actions present 

 him with all that is essential to the notion of Life. And in pur- 

 suing the history of the germ, from this, its simplest and most 

 homogeneous form, to the assumption of that completed and per- 

 fected type which is marked by the extreme heterogeneousness of 

 its different parts, he has another illustration of that law of pro- 

 gress from the general to the special, which is one of the highest 

 principles yet attained in the science of Vitality. 



