98 INTRODUCTION. 



to their effects, stimuli differ materially from each other. As 

 to the subjects on which they act, their difference is not less, 

 and depends on age, sex, and above all, on the diversity of the 

 organs, which feel more or less the action of the same agent. 

 Every thing being united in the organization, the action of 

 an organ is not isolated: those which have centres, influence 

 all that are subordinate to them. Others perform their func- 

 tions by association. Some of them to supply the want of it, 

 execute the action which is interrupted in another. There is 

 not a single one of them, which, being extraordinarily excited, 

 by an appropriate stimulus, does not influence more or less the 

 whole organism. 



OF THE DEVELOPMENT AND DIFFERENCES OF THE ORGANIZA- 

 TION. 



104. Each organ, each action, and consequently, the entire 

 organism, presents stadia or degrees of development and per- 

 fection. A first period is that of youth, of a successive growth 

 and perfection, or a second, very short one, is that wherein 

 the organization remains in a state of maturity; a third is that 

 in which the organization is progressively altered, and natural- 

 ly arrives at death and destruction. 



- 105. It is in the beginning of life, that the similitude be- 

 tween the lateral parts is the greatest. The heart is then ver- 

 tical and median, the lobes of the liver nearly equal, the 

 stomach vertical, etc. The upper and lower members are ex- 

 actly alike, at the moment they make their appearance, and for 

 a short time after. The genital organs of both sexes are at first 

 similar. It is also in the commencement of life that animals 

 bear the strongest resemblance to each other. The relative 

 size of the parts changes with age: thus the nervous system, 

 the senses, the heart, the liver the kidneys etc, are at first 

 large, in comparison with the rest of the body, while on the 

 contrary, the intestine, the spleen, genital organs, lungs, mem- 

 bers, etc, are very small as regards the rest of the body, and 

 the other organs. This added to the fact, that certain parts 

 disappear, or greatly diminish with age, constitutes a species 



