296 ZOOLOGICAL PHILOSOPHY 



ditions, we may be able to conceive how the parts of the system were 

 formed, and how they were filled with the subtle fluid which moves 

 within them and enables them to carry out their functions. 



We may suppose that when nature had advanced so far with animal 

 organisation that the essential fluid of animals had become highly 

 animalised, and the albumino-gelatinous substance been formed, this 

 substance would be secreted from the animal's chief fluid (blood, or 

 its substitute) and deposited in some part of the body : now observation 

 shows that this first occurs in the shape of several small separate masses, 

 and afterwards as a larger mass which becomes lengthened into a 

 ganglionic cord and occupies nearly the whole length of the body. 



The cellular tissue is modified by this mass of albumino-gelatinous 

 substance, and so provides it with its investing sheath, and that of its 

 various prolongations or threads. 



Now on examining the visible fluids which move or circulate in the 

 bodies of animals, I find that, in the animals with the simplest organisa- 

 tions, these fluids are much less complex and contain much fewer 

 principles than is the case in the more perfect animals. The blood of 

 a mammal is a more complex and animaUsed fluid than the whitish 

 serum of insects ; and this serum again is a more complex fluid than 

 that watery matter which moves in the bodies of polyps and 

 infusorians. 



This being so, I am justified in the beUef that those invisible and 

 uncontainable fluids which keep up irritabihty and vital movements 

 in the most imperfect animals, are the same as those existing in animals 

 with a highly complex and perfected organisation. In the latter, 

 however, they undergo so great a modification as to be changed into 

 containable fluids, though still invisible. 



It appears indeed that an invisible and very subtle special fluid, 

 which is modified during its presence in the blood of animals, is con- 

 tinually separating out to spread through the nervous medullary 

 masses, and incessantly makes good the wastage due to the various 

 activities of this system of organs. 



The medullary pulp of the nervous system, and the subtle fluid 

 moving within it, will thus only be formed when the complexity of 

 animal organisation has reached a sufficient development for the manu- 

 facture of these substances. 



Just as the internal fluids of animals are progressively modified, 

 animalised and compounded in correspondence with the progress in 

 the complexity and perfection of organisation ; so too the organs and 

 solid or containing parts of the body are gradually compounded and 

 diversified in the same way and by the same cause. Now the nervous 

 fluid, which becomes containable after its secretion by the blood, 



