
270 THE AMERICAN NATURALIST. [Vor. XXXVI. 
The chief object of this communication is to enforce the rec- 
‘ognition of homoplasy as something which must be accounted 
for. These homoplastic cusps do not arise from selection out 
of fortuitous variations, because they develop directly and are 
not picked from a number of alternates. Neither: 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 tritubercular 
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 like- 
ness of material to begin with.” 
Philosophically, predeterminate variation and evolution brings 
us upon dangerous ground. If all that is evolved in the Ter- 
tiary molar tooth is included in a latent or potential form, in 
the Cretaceous molar tooth we are nearing the embottement 
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 intermediate between 
homology 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 (é.e., homogeny). 
