284 STUDIES IN THE EVOLUTION OF ANIMALS 



the other there may be compound teeth, but also because there 

 may be a ' reverse ' to the phenomenon described, viz., that a large 

 complicated tooth, such as we see in certain mammals, may be a 

 fusion of a number of simple teeth ! And this is exactly what may 

 have happened, either suddenly, or by slow steps, when a long jaw 

 with many teeth passed into a short jaw with few teeth. Some of 

 the teeth may have been suppressed, but a number may have 

 become fused into one large compound tooth. 



We do not know what influences and controls the disposition 

 of the parts before the nerve-centres of the embryo are formed. It 

 does not follow, because there are yet no nerve-centres and 

 nerves, that therefore there is not something that controls the 

 cells and acts as a means of communication. 



Anyhow, when in the animal embryo the nervous system has 

 once taken form, we cannot have much doubt that it controls and 

 regulates the progress of the embryo. Its controlling influence in 

 the multiplication and disposition or arrangement of the body 

 cells, which are increasing by fission, may be likened to the 

 controlling influence of the battery or dynamo in electro-plating. 

 The two processes would appear to be parallel. We do not know 

 why some metals are deposited in a compact form, suited to 

 electro-plating, and why some are deposited in dendritic or tree- 

 like forms. All we know at present is that they do so. And all 

 we can say of monstrosities is that they do occtcr during the 

 evolution of the embryo, although from recent experiments in 

 artificial teratogeny we may hope at no distant period to learn 

 something more definite about the intimate causes of monstrosities. 



Some might say — ' This may be an ingenious stretch of thought, 

 which would somewhat explain aberrations in animals ; but what 

 about analogous aberrations in plants? We have no nervous 

 system there.' No, but 'nervous system' may be only words 



