There were apparently two main types of structure by means of which this mobility and elasticity could be attained, the Arthropod type and the Vertebrate type. There seems little to choose between them if we had to select one of them in order to obtain a highly mobile organic mechanism. Arthropod and Vertebrate seem to be equally complex if we take account of difference in size and the additional bodily mechanism that great size must involve. Certainly the musculature of the Vertebrate is more complex than in the Arthropod. But greater weight must require larger and more powerful muscles if the same degree of mobility relative to the size of the animal is to be attained, and this more complex musculature must carry with it a more complex brain. It must also be concomitant with a more massive skeleton, for rigid supports for the muscles must be present in the mechanism. Why are there no great insects or crustaceans? Mr Wells has suggested in one of his novels the formidability of a wasp two feet long! Such a creature would indeed be more dreadful than any predatory bird that we know if its activity were also that of the wasps that we know, just as a Copepod as large as a shark would be a more formidable animal than the fish. It seems possible that the reason for the smaller size of the Vertebrate is to be found in the nature of the skeleton. Powerful muscles would require a very strong and thick carapace, and this would attain a mass in a very large insect or crustacean which would require too much energy for its rapid transport. A rigid exoskeleton like that of an Arthropod also means that growth must take place by a process of ecdysis, that is, the animal grows only during the periods when it casts its shell; and the necessity of this process of ecdysis must be a formidable disadvantage in the case of a very large animal, if indeed it would be possible at all. Thus the Arthropod developing an exoskeleton must remain small, and this smallness, fortunately for the Vertebrate, has made it the less formidable animal. It was an accident of evolution that the Arthropods developed an exoskeleton instead of an endoskeleton. Undoubtedly the internal skeleton of the Vertebrates, with its light, hollow, cancellated bones, was mechanically the best means for the attachment of muscles. It made possible a greater degree of freedom of movement of the parts of the body, greater variety and plasticity of action, and it removed, to some extent, the limit of size and the embarrassing discontinuity of growth by ecdysis, with all the dangers that this involves. Above all, it led to the increased complexity of the central nervous system, since this became bound up with the increasing variety of bodily movement.