42 - Mr. E. R. Lankester on the use of 
thobdellea with those of Chetopods; but zoologists would he- 
sitate to assert homology in these cases, and it certainly seems 
improbable that there 1s homogeny. What Mr. Spencer calls 
“ superinduced segmentation," hitherto included by many 
zoologists as serial homology, falls under simple homoplasy, 
the detailed resemblances of the vertebre being thus explained, 
though it is possible that there is an obscured homogenous 
segmentation indicated in the earliest stages of vertebrate 
development. 
trust now to have said sufficient to illustrate the distinc- 
tion which I wish to draw between homogeny and homoplas 
and to have shown a probability that a good deal of the latter 
has been associated with the former under one head, “ homo- 
logy.” It is less likely to cause confusion if we have a new 
term than if we amend an old one, which is my reason for not 
retaining “homology.” It is not improbable that homoplasy 
may admit of further analysis; but it 1s sufficient here to dis- 
tinguish it from homogeny. I do not propose to defend 
against criticism the cases I have used in illustration. The 
views suggested with regard to particular cases are open to 
much discussion, and the views alluded to as being commonl 
held may in some instances be not very widely prevalent. 
This, however, does not affect the matter in hand. Concrete 
cases are given merely with a view to illustration, and to 
render clear what is the relative significance of the terms 
* homology,” “ homogeny,” and “ homoplasy." 
What is put forward here is this,—that under the term 
“homology,” belonging to another philosophy, evolutionists 
have described and do describe two kinds of agreement—the 
one, now proposed to be called *homogeny;" depending simply 
on the inheritance of a common part, the other, proposed to be 
called *homoplasy;" depending on a common action of evoking 
causes or moulding environment on such homogenous parts 
or on parts which for other reasons offer a likeness of materia 
to begin with. In distinguishing these two factors of a com- 
mon result we are only recognizing the principle of a plurality 
of causes tending to a common end, which is elsewhere recog- 
nizable and has been pointed out in biological phenomena. 
The explanation of the phenomena by the one law o 
logy is a part of that tendency to view Nature as more simple 
and more easily mastered than she really is, against which 
Bacon cautions us. 
