104 ZOOLOGICAL PHILOSOPHY 



chdefly among the invertebrate animals that the extinction of special 

 organs occurs. 



It is true that even before leaving the division of vertebrates, we 

 already witness great changes in the perfection of organs ; while some 

 even disappear altogether, such as the urinary bladder, the diaphragm, 

 the organ of voice, the ey ;lids, etc. The lung, for instance, which is 

 the most perfect respiratory organ, begins its degradation in the reptiles 

 and ceases to exist in the fishes, not to reappear again in any inver- 

 tebrate animal. Finally, the skeleton, the appendages of which 

 constitute the basis of the four extremities or limbs possessed by most 

 vertebrates, begins its deterioration mainly in the reptiles and comes 

 entirely to an end with the fishes. 



But it is in the division of invertebrates that the extinction takes 

 place of the heart, brain, gills, conglomerate glands, vessels for circula- 

 tion, the organs of hearing and sight, that of sexual reproduction 

 and even that of feeling, as also of movement. 



As I have already said, we should vainly seek in a polyp, such as the 

 hydra or most animals of that class, the slightest vestiges either of nerves 

 (organs of feeling) or of muscles (organs of movement) ! Irritability, 

 with which every polyp is highly endowed, alone replaces the faculty of 

 feehng which no polyp possesses, since it has not the essential organ for it. 



It also replaces the faculty of voluntary movement, since all will 

 is an act of the organ of intelligence, and this animal is absolutely 

 destitute of any such organ. All its movements are necessary results 

 of the impressions on its irritable parts of external stimuli ; and they 

 are carried out without any scope for choice. 



Put a hydra in a glass of water, and set this glass in a room where 

 daylight only enters by one window and therefore only from one side. 

 When this hydra has fixed on some point of the sides of the glass, 

 turn the glass so that the light strikes it on the opposite side to that 

 where the animal is. You will then always see the hydra go with 

 a slow movement and place itself where the light strikes, and stay there 

 so long as you do not change this point. This is the same in those parts 

 of plants which, without any act of will, lean towards the side from 

 which the light comes. 



Doubtless wherever a special organ no longer exists, the function 

 which it supports also ceases to exist ; and we may furthermore 

 clearly observe that according as an organ is degraded and reduced, 

 the function resulting from it becomes proportionally more vague 

 and imperfect. Thus, we find that on descending from the most 

 complex towards the simplest, the insects are the last animals which 

 have eyes ; but there is sound reason for the belief that they see very 

 dimly and make little use of them. 



