62 THE ORGANIC GROWTH OF THE LIVING WORLD sec. it 



characters ought not to surprise us. These somites have 

 arisen by the subdivision of a whole, and the first has the 

 same relation to the second as the second to the third, and so 

 on. !N"ow, since in lizards, as in caterpillars {xid. subsequent 

 sections), and also in mammals and birds, new peculiarities of 

 marking first arise on the hinder part of the body and extend 

 thence forwards, the question at once occurs whether this fact 

 harmonises with the explanation given for plants and 

 Ammonites, i.e. whether the hinder part in these animals is to 

 be regarded as the youngest. It is to be noted that a ver- 

 tebrate is segmented, and that there is much to be said for 

 the supposition that the vertebrates were once forms similar 

 to segmented worms ; on various grounds they are regarded 

 by many as descended from the latter. The Arthropoda, 

 including caterpillars, have similar relations to the segmented 

 worms (Annelids). But in the latter the most anterior 

 segments obviously as a matter of fact are composed of the 

 oldest tissues. This is proved by the fact of ontogeny : so far 

 as special attention has hitherto been paid to the commence- 

 ment and succession of segmentation in Annelid larvae, it has 

 shown that the process begins in the anterior part of the 

 body and is continued thence backwards ; the worm must 

 therefore grow at the posterior end. Exactly the same thing 

 is observed in our fresh- water worm, Xais probescidea. In 

 the young worms produced by division the new head always 

 consists of the oldest material — growth takes place at the 

 posterior end. 



At present, my only intention in referring to this subject 

 is to indicate a definite point of view from which to investigate 

 a question which requires a work to itself. Moreover, I am 

 not at present in a position to explain in a similar way the 

 infero-superior modification of the marking wliich takes place 

 in many mammals. 



