DEGRADATION OF ORGANISATION 81 



Tace has been forced to contract by the conditions in which it is 

 placed. 



Hence we see, on the one hand, that if vertebrates differ markedly 

 from one another in their organisation, it is because nature only started 

 to carry out her plan in their respect with the fishes ; that she made 

 further advances with the reptiles ; that she carried it still nearer 

 perfection with the birds, and that finally she only attained the end 

 with the most perfect mammals. 



On the other hand, we cannot fail to recognise that if the perfection 

 of the plan of organisation of the vertebrates does not everywhere 

 show a regular and even gradation from the most imperfect fishes 

 to the most perfect mammals, the reason is that nature's work has 

 often been modified, thwarted and even reversed by the influence 

 exercised by very different and indeed conflicting conditions of life 

 upon animals exposed to them throughout a long succession of 

 generations. 



Annihilation of the Vertebral Column. 



On reaching this point in the animal scale the vertebral column 

 becomes entirely annihilated. Since this column is the basis of 

 every true skeleton, and since this bony framework is an important 

 part of the organisation of the most perfect animals, it follows that all 

 the invertebrate animals, which we are about to investigate in turn, 

 must have an organisation still more degraded than that of the four 

 classes that we have just passed in review. Henceforth, therefore, 

 the supports for muscular activity will no longer reside in any internal 

 parts. 



Moreover, none of the invertebrate animals breathes by cellular 

 lungs ; none of them has any voice nor consequently any organ 

 for this faculty ; finally they mostly appear devoid of true blood, 

 that is to say, of that fluid which in the vertebrate is essentially red, 

 but which only owes its colour to the intensity of their animalisation, 

 and proves especially a real circulation. How grave an abuse of words 

 it would be to give the name of blood to the thin and colourless fluid 

 which moves slowly through the cellular substance of the polyps ! 

 We might as well apply the name to the sap of plants. 



Be-sides the vertebral colimin, we also lose here the iris which is 

 characteristic of the eyes of the most perfect animals ; for such of the 

 invertebrates as have eyes have no distinct irises. 



Kidneys in the same way are only foimd among the vertebrates, 

 and fishes are the last animals where this organ is met with. Hence- 

 forward there is no more spinal cord, no more great sympathetic 

 nerve. 



