OcTOBER 2, 1913] 
NATURE 
147 
extracting the greatest common measure out of the 
unknown number of causes which combine in the 
production of even the apparently simplest organ. 
These resemblances are a very promising field, and 
the balance of importance will more and more incline 
towards the investigation of function, a study which, 
however, does not mean mere physiology with its 
" present-day aims in the now tacitly accepted sense, but 
that broad study of life and death which is to yield 
the answer to the question Why? 
Meantime, comparative anatomy will not be 
shelved; it will always retain the casting-vote as to 
the degree of affinity among resemblances, but 
emphatically its whole work is not to be restricted to 
this occupation. It will increasingly have to reckon 
with the functions, indeed never without them. The 
animal refuses to yield its secrets unless it be con- 
sidered as a living individual. It is true that Gegen- 
baur himself was most emphatic in asserting that an 
organ is the result of its function. Often he held up 
to scorn the embryographer’s method of muddling 
cause and effect, or he mercilessly showed that in the 
reconstruction of the evolution of an organ certain 
features cannot have been phases unless they imply 
physiological continuity. And yet how moderately is 
function dealt with in his monumental text-book and 
how little is there in others, even in text-books of 
zoology ! 
Habt alle die Theile in der Hand, 
Fehlt leider nur das geistige Band—Life ! 
We have become accustomed to the fact that like 
begets like with small differences, and from the 
accepted point of view of evolution versus creation 
we no longer wonder that descendants slowly change 
and diverge. But we are rightly impressed when unlike 
comes to produce like, since this phenomenon seems 
to indicate a tendency, a set purpose, a beau idéal, 
which line of thought or rather imperfect way of 
expression leads dangerously near to the crassest 
teleology. 
But, teleology apart, we can postulate a perfect 
agreement in function and structure between creatures 
which have no community of descent. The notion 
that such agreement must.be due to blood-relationship 
involved, among other difficulties, the dangerous con- 
clusion that the hypothetical ancestor of a given 
genuine group possessed in potentiality the Anlagen 
of all the characters exhibited by one or other of the 
component members of the said group. 
The same line of thought explained the majority of 
human abnormalities as atavistic, a procedure which 
would turn the revered ancestor of our species into 
a perfect museum of antiquities, stocked with tools 
for every possible emergency. 
The more elaborate certain resemblances are, the 
more they seem to bear the hall-mark of near affinity 
of their owners. When occurring in far-related groups 
they are taken at least as indications of the homology 
of the organs. There is, for instance, a remarkable 
resemblance between the bulla of the whale’s ear and 
that of the Pythonomorph Plioplatycarpus. If you 
homologise the mammalian tympanic with the 
quadrate the resemblance loses much of its perplexity, 
and certain Chelonians make it easier to understand 
-how the modifications may have been brought about. 
But, although we can arrange the Chelonian, Pythono- 
morph, and Cetacean conditions in a progressive line, 
this need not represent the pedigree of this bulla. Nor 
is it necessarily referable to the same Anlage. Lastly, 
if, as many anatomists believe, the reptilian quadrate 
appears in the mammals as the incus, then all homo- 
logy and homogeny of this bullae is excluded. In 
either case we stand before the problem of the forma- 
tion of a bulla as such. The significant point is’ this, 
that although we dismiss the bulla of whale and reptile ! 
NO. 2292, VOL. 92] 
| as obvious homoplasy, such resemblances, if they occur 
in two orders of reptiles, we take as indicative of rela- 
tionship until positive evidence to the contrary is 
produced. That this is an unsound method is brought 
home to us by an ever-increasing number of cases 
which tend to throw suspicion on many of our recon- 
structions. Not a few zoologists look upon such cases 
as a nuisance and the underlying principle as a bug- 
bear. So far from that being the case, such study 
promises much beyond the pruning of our standard 
trees—by relieving them of what reveal themselves as 
grafts instead of genuine growth—namely, the revela- 
tion of one or other of the many agencies in ‘their 
growth and structure. 
Since there are all sorts and conditions of re- 
semblances, we require technical terms. Of these there 
is abundance, and it is with reluctance that I propose 
adding to them. I do so because unfortunately some 
terms are undefined, perhaps not definable; others 
have not ‘‘caught on,’’ or they suffer from that mis- 
chievous law of priority in nomenclature. 
The terms concerning morphological homologies 
date from Owen; Gegenbaur and Haeckel re- 
arranged them slightly. Lankester, in 1870, intro- 
duced the terms homogenous, meaning alike born, 
and homoplastic, or alike moulded. Mivart rightly 
found fault with the detailed definition and the sub- 
divisions of homoplasy, and very logically invented 
dozens of new terms, few of which, if any, have sur- 
vived. It is not necessary to survey the ensuing 
literature. For expressing the same phenomenon we 
have now the choice between homoplasy, homomorphy, 
isomorphy, heterophyletic convergence, parallelism, 
&c. After various papers by Osborn, who has gone 
very fully into these questions, and Willey’s ‘* Para- 
llelism,’”’ Abel, in his fascinating ‘‘Grundztige der 
Palzobiologie,’’ has striven to show by numerous 
examples that the resemblances or ‘‘adaptive forma- 
tions” are cases of parallelism if they depend upon 
the same function of homologous organs, and con- 
vergences if brought about by the same function of 
non-homologous organs. 
I suggest an elastic terminology for the various 
resemblances indicative of the degree of homology of 
the respective organs, the degree of affinity of their 
owners, and lastly the degree of the structural like- 
ness attained. 
Homogeny.—The structural feature is invented once 
and is transmitted, without a break, to the descen- 
dants, in which it remains unaltered, or it changes by 
mutation or divergence, neither of which changes can 
bring the ultimate results nearer to each other. Nor 
can their owners become more like each other, since 
the respective character made its first appearance 
either in one individual, or, more probably, in many 
of one and the same homogenous community. 
Homoplasy.—The feature or character is invented 
more than once, and independently. This phenomenon 
excludes absolute identity; it implies some unlikeness 
due to some difference in the material, and there is 
further the chance of the two or more inventions, and 
therefore also of their owners, becoming more like 
each other than they were before 
CaTEGORIES OF HOMOPLASY. 
Isotely.\—If the character, feature, or organ has 
been evolved out of homologous parts or material, as 
is most likely the case in closely related groups, and 
if the subsequent modifications proceed by similar 
stages and means, there is a fair probability or chance 
of very close resemblance. Jso-tely: the same mark 
has been hit. 
Homoeotely.—Although the feature has been evolved 
1 Cf “Isotely and Coralsnakes.” By H. Gadow, Zoolog. Jahrbiither, 
Abt. f. Syst., xxxi., rgrt. 
