368 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



into vertebrae, by supposing it to be due to the lateral 

 swimming movements of the fishes, which first exhibit 

 these structures.^ With this view various later authors 

 have agreed, and I have offered some additional evi- 

 dence of the soundness of this position with respect to 

 the vertebral axis of Batrachia,^ and the origin of limb 

 articulations.' It is true that the origin of segmenta- 

 tion in the vertebral column of the true fishes and the 

 Batrac*hia turns out to have been less simple in its pro- 

 cess than was suggested by Mr. Spencer, but his gen- 

 eral principle holds good, now that paleontology has 

 cleared up the subject. 



The Echinodermata, Mollusca, Arthropoda, and 

 Vertebrata possess external or internal calcareous or 

 chitinous skeletons for the most part. The lower forms 

 of all these branches, however, are more or less deficient 

 in this kind of protection, and embryology indicates 

 that all of them are the descendants of the Vermes or 

 worms, which are mostly without such hard supports 

 and protections. Whether this be demonstrated or 

 not, we have plenty of evidence to show that the prim- 

 itive Vertebrata were without hard skeletons, and that 

 their bodies were composed internally and externally 

 of perfectly flexible tissues. 



If we now imagine that either the integuments, or 

 an axial rod, of a worm-like animal has become the seat 

 of a calcareous or chitinous deposit, it is evident that 

 the movements of the animal in swimming or creeping 

 must have interrupted the deposit at definite points of 

 its length. The lateral flexure of the body would be 

 restricted to certain points, and the intervening spaces 



"^ Principles of Biology , 1873, pp. 198-204. 



2 Origin of the Fittest, 1887, p. 305. 



3 Mechanical Causes of Origin of Hard Parts of Mamtnalia, 1889, p. 163. 



