96 AN INTRODUCTION TO SCIENCE 



favoured it much in the struggle for existence. Of its suc- 

 cessors but few survived, and these only such species as 

 inclined strongly towards the vertebrate type. The new 

 type was, therefore, established with comparative rapidity. 

 But when once it had acquired something like permanent 

 character this form of animal showed that it could not only 

 hold its own against invertebrates, but that it contained a 

 potentiality for development into "a great nation." It is 

 difficult to imagine the conditions which favoured this 

 remarkable transition. Probably it is better not to try. 

 The facts remain that, whereas it can be proved from em- 

 bryological evidence that the vertebrate had an invertebrate 

 ancestor,iand whereas the difference between the two types 

 is of the most pronounced kind, zoologists are not agreed 

 that any indubitably intermediate forms have been found, 

 either extant or extinct. 



Anyone who has taken the facts above stated into con- 

 sideration will anticipate a bold theory of the transition from 

 an invertebrate to a vertebrate type, but the more he dwells 

 upon the essential differences between the two the more 

 clearly will he see that only a bold theory can hope to 

 justify itself. The most striking differences are these : The 

 vertebrate has a backbone which gives off two series of bony 

 arches, the one dorsal (the vertebral arches) to enclose the 

 spinal cord, the other ventral jaws, hyoid arch and ribs 

 to enclose the alimentary canal and viscera. When an in- 

 vertebrate has a skeleton it is usually external, like the 

 calcareous case of a lobster, for example. The vertebrate 

 central nervous system (brain and spinal cord) lies entirely 

 on the dorsal side of the vertebral column, and therefore on 

 the dorsal side of the alimentary canal. The central nervous 

 system of an invertebrate is partly dorsal, partly ventral. In 

 the octopus, for example, which (with the exception of 

 spiders and scorpions, perhaps) has the nearest approach to 

 a brain found in any invertebrate, the nervous ganglia are 

 collected into a group, enclosed by a rudimentary cartilagi- 



