Of late years attempts have been made to speculate about the origin of the Myriapods—that is, to endeavour to obtain by means of investigation of their anatomy, embryology, and palaeontological history, some idea of the history of the group. Such attempts at research into the _phylogeny_, as it is called, of a group must be more or less speculative until our knowledge is much greater than {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 Pauropoda. The other theory holds that the Myriapods, as well as the Insecta, have been derived from some ancestor bearing a resemblance 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 formation of the body segments, the mesoblastic segmentation, and the origin of the various organs of the body.