CUSP KK<TH!KAI>ATin\S 237 



The elements to which these terms are applied are lu-st exemplified 

 in the molar teeth of some of the primitive horses ! Fig. LMTtj. 



The teeth are by no means the only structure* which evolve 

 under this principle, the skull, vertebral column, and limbs also 

 evolving under it more or less completely: but the teeth ;iltord a 

 singularly beautiful illustration of it because they exclude individual 

 modification. 



The chief object of this communication is to enforce the recognition 

 of homoplasy as something which must be accounted for. These 

 homoplastic cusps do not arise from selection out of fortuitous varia- 

 tions, because they develop directly and are not picked from a number 

 of alternates. Xeither does it appear that the mechanical- inheritance 

 theory, if granted, would produce such a remarkable uniformity of 

 result. We are forced to the conclusion that in the original trituber- 

 cular constitution of the teeth there is some principle which unifies 

 the subsequent variation and evolution up to a certain point. Herein 

 lies the appropriateness of Lankester's phrase, "a likeness of material 

 to begin with." 



Philosophically, predeterminate variation and evolution brings us 

 upon dangerous ground. If all that is evolved in the Tertiary molar 

 tooth is included in a latent or potential form, in the Cretaceous molar 

 tooth we are Hearing the emboitement hypothesis of Bonnet or the 

 archetype of Oken and Owen. Embryologists have recently gotten 

 into the same dilemma, and my colleague, Wilson, has proposed to 

 drop the idea " homology ' altogether and substitute the idea 

 "equivalent." In the present case, however, I think we have to deal 

 with homology, or, more strictly, with a principle intenin(!in/< I, firm/ 

 limnology and analogy. 



In a paper recently read before the American Morphological Society 

 (December, 1901), this author has urged the necessity of adhering 

 as closely as possible to the historical standard in the embryological 

 study of homology, and of avoiding the use of the term " homology ' 

 when this standard is not available. He therefore suggests for 

 descriptive purposes the use of the non-committal terms "equivalent" 

 and " homoblastic," the former being applied to embryonic structures 

 of like fate (i.e., giving rise to homologous parts), the latter to those 

 of like embryonic origin. The only decisive test of the homology is 

 historic community of derivation (i.e., homogeny). 



