ii GENERAL CONCLUSIONS 79 



it is at present. But such inquiries have their value, and the 

 schemes of descent and phylogenetic trees, at any rate, indicate a 

 real relation to different groups, even if they do not provide us 

 with a real and actual history of the animals. 



There have been two main theories about the descent of the 

 Myriapoda. One of these derives them directly from the Insecta 

 through the forms known as the Thysanura, which resemble in 

 such a degree the Myriapod Orders of Symphyla and Pauro- 

 poda. The other theory holds that the Myriapods, as well as the 

 Insecta, have been derived from some ancestor bearing a resem- 

 blance to Peripatus. In other words, one theory claims that the 

 relationship of Myriapoda to Insecta is that of father and son ; 

 the other that the relationship between the two is that of 

 brother to brother. The arguments by which these theories are 

 respectively supported consist for the most part of an analysis of 

 the different characters of the anatomy and embryology and the 

 determination of the most primitive among them. For example, 

 the supporters of the theory that the Thysanura are the most 

 nearly allied to the Myriapod ancestor lay great weight on the fact 

 that some Myriapods are born with three pairs of legs only, and 

 they compare this stage in the life history of the Myriapoda to the 

 metamorphosis and larval stage of Insects. For the supporters 

 of this view the Orders of Symphyla and Pauropoda are the 

 most primitive of the Myriapods. On the other hand, the 

 followers of the other theory do not allow that the characters 

 in which the Myriapods are like Insects are primitive ones, but 

 they lay more stress on the characters found in the early 

 development, such as the character of the process of the forma- 

 tion of the body segments, the mesoblastic segmentation, and the 

 origin of the various organs of the body. 



It may be easily understood that such differences in the 

 estimation of the primitive characters of the embryology of a 

 group may arise. Embryology has been compared by one of the 

 greatest of modern embryologists to " an ancient manuscript with 

 many of the sheets lost, others displaced, and with spurious 

 passages interpolated by a later hand." What wonder is it that 

 different people examining such a record should come to different 

 conclusions as to the more doubtful and difficult portions of 

 it. It is this very difficulty which makes the principal interest 

 in the study, and although our knowledge of the language in 



