82 ZOOLOGICAL PHILOSOPHY 



A final very important observation is that among vertebrates, and 

 especially in the neighbourhood of that extremity of the animal scale 

 where the most perfect animals are found, all the essential organs 

 are isolated or have each an isolated seat in as many special places. 

 We shall soon see that the complete contrary holds good according 

 as we approach the other extremity of the scale. 



It is then obvious that all the invertebrate animals have a less 

 perfect organisation than any of those which possess a vertebral 

 column ; while the organisation of mammals is that which from all 

 aspects includes the most perfect animals and is beyond question 

 the true type of the highest perfection. 



Let us now enquire whether the classes and large families into 

 which the long series of invertebrate animals is divided also exhibit, 

 when we compare them together, an increasing degradation in the 

 complexity and perfection of their organisation. 



INVERTEBRATE ANIMALS. 



On reaching invertebrate animals we enter upon an immense series 

 of diverse creatures, the most numerous of any existing in nature, 

 the most curious and interesting with regard to the variations observed 

 in their organisation and faculties. 



On observing their condition, we are convinced that in bringing 

 them successively into existence, nature has proceeded gradually 

 from the simplest to the most complex. Now since the purpose 

 in view has been to attain a plan of organisation which should admit 

 of the highest perfection (that of the vertebrates) — a plan very 

 different from those which nature had hitherto used to reach this point 

 — we may be sure that among these numerous animals we shall not 

 meet with a single system of organisation progressively perfected, 

 but with various quite distinct systems, each one taking its start 

 at the point where each organ of highest importance began to exist. 



For instance, when nature attained to the creation of a special organ 

 for digestion (as in the polyps) she then gave for the first time a special 

 constant shape to the animals provided with it ; seeing that the in- 

 fusorians with which she began everything could not possess either 

 the faculty endowed by this organ, or the kind of shape and organisa- 

 tion favourable to its functions. 



She subsequently established a special organ for respiration, and 

 in proportion as she varied this organ in order to perfect it and to 

 accommodate it to the animal's environment, she diversified their 

 organisation, in so far as the existence and development of the other 

 special organs rendered it necessary. 



