SERIAL DEVELOPMENT. 159 



system and a digestive system, both in the lowest possible form, 

 and at the beginning no more definitely fixed than in the Infuso- 

 rian. Then in the second place these organs begin to have defi- 

 niteness, but yet of that loose kind which we find in Polyps and 

 Hydras ; the digestive and circulatory systems are not separated ; 

 so it is in Polyps, Corals, and jelly-fishes ; but developing further, 

 the digestive and circulatory systems appear partially distinct, as 

 in Echini and starfishes; then more so, as in shell-fish, (Mol- 

 lusca,) and at the same time the nervous system becomes prom- 

 inent in a great ganglion, the brain, (curiously enough, too, re- 

 sembling the nervous system of the highest Mollusca in propor- 

 tions,) which extends backwards, sending out as it were, toward 

 the tail, a ribbon-like projection along the middle line of the 

 body, as in the highest kinds of Insecta. 



But here the relation stops, and here it is that the older em- 

 bryologists mistook their way ; for, notwithstanding these paral- 

 lelisms of growth which I have traced out, they have altogether 

 different relations to each other in the rabbit from what obtain 

 in the infusorian, or polyp, or starfish, or shell-fish, or insect. 



This difference in the relation of parts is the dividing line. 

 As I told you in another place, in considering the typical forms of 

 life, it is the relation and not the nature of a substance which is to 

 be taken into account. Relation should be the ruling standard. 



In the rabbit, notwithstanding the trend or course which it 

 pursues along the animal-base-line, the relations into which 

 its gradually developing organs are brought are different from 

 those in which the organs of an infusorian, or polyp, or mol- 

 luscan, or insect are arranged when developing; and all four of 

 these last are as different in this respect from each other as they 

 are from the vertebrate rabbit. 



But in the ideal diagrams of the types you do not see these 

 relations exemplified, so that you could detect them. In fact, I 

 might say that I have in a measure concealed them by the posi- 

 tion in which the figures are placed, and by giving prominency 

 to the main idea, bilaterality. 



But I did this, as I have just said, that I might represent a 



