146 
(Spencer), has taken place; he stated the problem of 
comparative anatomy as the reduction of the differ- 
ences in the organisation of the various animals to 
a common condition; and as homologous organs he 
defined those which are of such a common, single 
origin, His first work in this new line is his classical 
treatise on the Carpus and Tarsus (1864). 
It followed from this point of view that the degree 
of resemblance in structure between homologous 
organs and the number of such kindred organs pre- 
sent is a measure for the affinity of their owners. 
So was ushered in the era of pedigree of organs, of 
functions, of the animals themselves. The tracing 
of the divergence of homogenous parts became all- 
important, whilst those organs or features which re- 
vealed themselves as of different origin, and therefore 
as analogous only, were discarded as misleading in 
the all-important search for pedigrees. Functional 
correspondence was dismissed as ‘‘ mere analogy,’’ and 
even the systematist has learnt to scorn these so-called 
physiological or adaptive characters as good enough 
only for artificial keys. A curious view of things, 
just as if it was not one and the same process which 
has produced and abolished both sets of characters, 
the so-called fundamental or ‘reliable,’ as well as 
the analogous. : 
As A. Willey has put it happily, there was more 
rejoicing over the discovery of the homology of some 
unimportant little organ than over the finding of the 
most appalling unrelated resemblance. Morphology 
had become somewhat intolerant in the application of 
its canons, especially since it was aided by the pheno- 
menal growth of embryology. You must not com- 
pare ectodermal with endodermal products. You must 
not make a likeness out of another germinal layer or 
anything that appertains to it, because if you do 
that would be a horror, a heresy, a homoplasy. 
Haeckel went so far as to distinguish between a 
true homology, or homophyly, which depends upon 
the same origin, and a false homology, which applies 
to all those organic resemblances which derive from 
an equivalent adaptation to similar developmental 
conditions. And he stated that the whole art of the 
morphologist consists in the successful distinction be- 
tween these two categories. If we were able to draw this 
distinction in every case, possibly some day the grand 
tree of each great phylum, maybe of the whole king- 
dom, might be reconstructed. That would indeed be 
a tree of knowledge, and, paradoxically enough, it 
would be the deathblow to classification, since in this, 
the one and only true natural system, every degree 
of consanguinity and relationship throughout all 
animated nature, past and present, would be accounted 
for; and to that system no classification would be 
applicable, since each horizon would require its own 
grouping. There could be definable neither classes, 
orders, families, nor species, since each of these con- 
ceptions would be boundless in an upward or down- 
ward direction, 
Never mind the ensuing chaos; we should at least 
have the pedigree of all our fellow-creatures, and of 
ourselves among them. Not absolute proof, but the 
nearest possible demonstration that transformation has 
taken place. Empirically we know this already, since, 
wherever sufficient material has been studied, be it 
organs, species, or larger groups, we find first that 
these units had ancestors, and, secondly, that the 
ancestors were at least a little different. Evolu- 
tion is a fact of experience, proved by cir- 
cumstantial evidence. Nevertheless we are not 
satisfied with the conviction that life is  sub- 
ject to an unceasing change, not even with the 
knowledge of the particular adjustments. We now 
want to understand the motive cause. First, What, 
then How, and now Why? 
NO. 2292, VOL. 92] 
NATURE 
ee ee 
[OcTOBER 2, 1913 
| It is the active search for an answer to this ques- 
tion (Why?) which is characteristic of our time. More 
; and more the organisms and ‘their organs are con- 
sidered as living, functional things. The mainspring 
of our science, perhaps of all science, is not its utility, 
not the desire to do good, but, as an eminently 
matter-of-fact man, the father of Frederick the Great, 
asking for monetary help) in the following shockingly 
homely words: ‘‘Der Grund ist derer Leute ihre 
verfluchte Curieusiteit."". This blamed curiosity, the 
beginnings of which can be traced very far back in the 
lower animals, is most acutely centred in our desire 
to find out who we are, whence we have come, and 
whither we shall go. And even if zoology, consider- 
ing the first and last of these three questions as settled, 
should some day solve the problem: Whence have we 
come? there would remain outside zoology the greater 
Why ? 
Generalisations, conclusions, can be arrived at 
only through comparison. Comparison leads no fur- 
ther where the objects are alike. If, for instance, 
we restrict ourselves to the search for true homo- 
logies, dealing with homogenes only, all we find is 
that once upon a time some organism has produced, 
invented, a certain arrangement or Anlage out of 
which that organ arose, the various features of which 
we have compared in the descendants. Result: we 
have arrived at an accomplished fact. These things, 
in spite of all their variety in structure and function, 
being homogenes, tell us nothing, because according 
| to our mode of procedure we cannot compare that 
monophyletic Anlage with anything else, since we 
have reduced all the homogenous modifications to one. 
Logically it is true that there can have been only one, 
but in the living world of nature there are no such 
iron-bound categories and absolute distinctions. For 
instance, if we compare the organs of one and the 
same individual, we at once observe repetition, e.g. 
that of serial homology, which implies many difficul- 
ties, with very different interpretations. Even in such 
an apparently simple case as the relation between 
shoulder girdle and pelvis we are at a loss, since the 
decision depends upon our view as to the origin of 
the paired limbs, whether both are modified visceral 
arches, and in this case serially repeated homogenes, 
or whether they are the derivatives from one lateral 
fin, which is itself a serial compound, from which, 
however, the proximal elements, the girdles, are sup- 
posed to have arisen independently. What is meta- 
merism? Is it the outcome of a process of successive 
repetitions so ‘that the units are homogenes, or did 
the division take place at one time all along the line, 
or is it due to a combination of the two procedures ? 
The same vagueness finds its parallel when dealing 
with the corresponding organs of different animals, 
since these afford the absolute chance that organs of 
the same structure and function may not be reducible 
to one germ, but may be shown to have arisen inde- 
pendently in time as well as with reference to the 
space they occupy in their owners. As heterogenes 
they can be compared as to their causes. In the study 
of the evolution of homogenes the problem is to 
account for their divergencies, whilst the likeness, the 
agreements, so to speak their greatest common 
measure, is eo ipso taken to be due to inheritance. 
When, on the contrary, dealing with heterogenes, we 
are attracted by their resemblances, which since they 
cannot be due to inheritance must have a common 
cause outside themselves. Now, since a_ leading 
feature of the evolution of homogenes is divergence, 
whilst that of heterogenes implies convergence from 
different starting points, it, follows that the more 
distant are these respective starting points (either in 
time or in the material) the better is our chance of 
told his Royal Academicians (who, of course, were ° 
ES 
