89 
1916-17.] Experiments and Observations on Crustacea. 
“ When identical or nearly similar forces, or environments, act on two 
or more parts of an organism which are exactly or nearly alike, the re- 
sulting modifications of the various parts will be exactly or nearly alike. 
Further, if, instead of similar parts in the same organism, we suppose the 
same forces to act on parts in two organisms, which parts are exactly or 
nearly alike and sometimes homogenetic, the resulting correspondences 
called forth in the several parts in the two organisms will be nearly or 
exactly alike. ... I propose to call this kind of agreement homoplasis or 
homoplasy .” 
Lankester’s statement is exceedingly illuminating. If once we admit 
that in the term “ homology ” shall be included the idea of homoplasy, then 
the distinction between homology and analogy is purely relative ; for in 
the end any two animals may be traced back to a common ancestor, and 
the distinction then depends entirely on the nearness or remoteness of 
the consanguinity.* If we restrict the connotation of homology to what 
is implied in Lankester’s term homogeny, then the “ serial homologies ’’ 
are analogies. In view of this it would be much better to discard the 
two terms “ homology ” and “ analogy ” and to substitute therefor “ homo- 
geny ” and “ homoplasy.” 
Lucas has cogently remarked that the classical example of homology 
cited by Geoffroy St Hilaire, to whom the origin of the whole distinction 
is traceable, is no real example of a difference in function. “ He 
(Geoffroy) pointed out that the fore-legs of mammals and the pectoral 
fins of fishes correspond in structure, though they perform such diverse 
functions as running, climbing, and swimming.” To a modern physiologist 
with far deeper power of functional analysis, a fin and a walking leg 
may be essentially similar mechanisms. Indeed, it is difficult to cite any 
case of homogeny worth specifying as such, unless the function of the 
organs has undergone change, i.e. unless the case in question could not 
by any chance be taken as one of analogy ; which assertion will commend 
itself to any histologist. 
Lankester’s statement is equally illuminating in its positive aspect. 
Outside the inherent unfolding impulse in organised structure, acting 
upon it and moulding it, are external forces or environments, and the 
interaction between the two is pictured as being direct. Here lies the 
essence of the question of homoplasy , or, if we like, of analogy, and to 
this very conclusion I had been led before reading Lankester’s paper. 
Let us take the question in another way. 
There is a limited number of animal phyla, representing different 
* Lankester liimself would still discriminate between homoplasy and analogy. 
