574 THE POPULAR SCIENCE MONTHLY. 



From all of which results the general conclusion, worked out more 

 fully in my paper on the " Homologues of Organic Creatures," that here 

 is the true route of organic development ; the vertebrates from the 

 mollusks, and these from the articulates proceed ; and that the basis 

 of the history we are seeking must be searched for in the history of 

 segmeyitation. 



All are familiar with the construction of an annulose ring or seg- 

 ment. To the rest of the compound creature each segment is a mi- 

 crocosm, precisly repeating similar parts, or their homologues. To 

 each segment belongs a neural ganglion, or rather a pair of ganglia ; 

 a dorsal vessel swelling in the middle to a pulsating organ, or heart, 

 or rudiments thereof ; and each is furnished with stigmata, spiracles, 

 or appendages, which are homologous of each other, or, as before, with 

 rudiments thereol And all these by sufficient analysis are to be 

 traced as actually present in every true segment, however often the 

 function of the homologous structure may be changed, or however it 

 may be reduced to a rudiment, or atrophied, or absorbed and appar- 

 ently lost. These homologues constitute the basis of biology^ as 

 they do of the life-functions of all creatures made of segments, which 

 includes, as we have seen, mollusks and vertebrates, as well as annulosa 

 — possibly, also, molluscoidea ; unless the latter should be considered 

 a modified coelenterate, and then it would be the beginning and unit 

 of the series. If, as sometimes contended (and these are unsettled 

 questions), the molluscoid is an evolution of the annulose type, and 

 its embryonic history tends to prove this, then all the segments have 

 been lost but one. For the molluscoids are monosegmental. Nor is 

 this proposition strange or improbable. Undoubtedly such a reduc- 

 tion to one segment takes place in high orders of genuine mollusks,, 

 as in gasteropods (of which you have instances in common snails), 

 in whom one segment has become atroj)hied, leaving generally a rudi- 

 ment behind. 



In vertebrata, which more immediately concern us, two segments, 

 and only two, are always present, and always bear with them their 

 distinctive elements or appendages, however rudimentary some of the 

 latter may sometimes appear. These segments really constitute the 

 well-known bilateral arrangement of parts and organs so general in 

 animals of this class. Here, again, it is necessary to go back a little 

 in order that we may make the greater speed forward. It is neces- 

 sary distinctly to understand what we are talking about, and what we 

 mean by segments. In the discussion of the bisegmental organization 

 of vertebrates, the question comes up whether any true homologues 

 exist in the two great classes, vertebrata and annulosa. In the pres- 

 ent advanced condition of biology, this question receives a decided 

 affirmative. Until recently the contrary was generally supposed. 



"When it is seen that two sides of a vertebrate, if it be split asun- 

 der down the backbone, present exact counterparts of each other, 



