DEGRADATION OF ORGANISATION 93 



suddenly drop out and are really annihilated, since they are not found 

 again in the classes which remain to be considered. 



Disappearance op the Nervous System. 



Here for instance the nervous system (the nerves and their centre 

 of communication) completely disappears, and is no more found in 

 any of the animals of the succeeding classes. 



In the most perfect animals, this system consists of a brain which 

 appears to serve for carrying out acts of intelHgence. At its base is 

 the nucleus of sensations from which issue nerves and also a dorsal 

 spinal cord which sends out other nerves to various parts. 



Among the vertebrates the brain becomes regularly reduced, and 

 as its volume diminishes the spinal cord becomes larger and seems to 

 take its place. 



Among the molluscs which constitute the first class of the inver- 

 tebrates the brain still exists, but there is no spinal cord nor a gan- 

 glionic longitudinal cord, and as gangUa are rare the nerves do not 

 appear to have nodes. 



Lastly, in the five following classes the nervous system is approach- 

 ing its end and is reduced to the very small rudiments of a brain and 

 to a longitudinal cord from which issue nerves. Thereafter there is 

 BO longer a separate nucleus for sensations, but a multitude of small 

 nuclei scattered throughout the length of the animal's body. 



Hence among insects, the important system of feehng comes to an 

 «nd ; a system which at a certain stage of development gives birth 

 to ideas and which in its highest perfection can produce all the acts 

 of intelHgence ; which, lastly, is the source whence muscular action 

 derives its power and without which sexual reproduction apparently 

 could not exist. 



The centre of communication of the nervous system is situated 

 in the brain or at its base or in a ganglionic longitudinal cord. There 

 is still a longitudinal cord when there is no longer any obvious brain ; 

 but when there is neither a brain nor a longitudinal cord, the nervous 

 system ceases to exist. 



Disappearance of the Sexual Organs. 

 Here again all traces of sexual reproduction disappear ; indeed 

 among the animals about to be cited it is no longer possible to recognise 

 organs for true impregnation. We shall, however, still find among 

 the animals of the two following classes kinds of ovaries, abounding 

 in oviform corpuscles that are alleged to be eggs ; but I look upon 

 these supposed eggs which can develop without previous fertihsation 



