PRESIDENTIAL ADDRESS, 5O5 
The distinction between these three categories must be vague because that 
between homology and analogy is also arbitrary, depending upon the standpoint 
of comparison. As lateral outgrowths of vertebre all ribs are homogenes, but 
if there are at least hemal and pleural ribs then those organs are not homo- 
logous even within the class of fishes. If we trace a common origin far enough 
back we arrive near bedrock with the germinal layers. So there are specific, 
generic, ordinal, &c., homoplasies. The potentiality of resemblance increases 
with the kinship of the material. 
Bateson, in his study of Homeosis, has rightly made the solemn quotation : 
‘There is the flesh of fishes... birds... beasts, &c.’ Their flesh will not 
and cannot react in exactly the same way under otherwise precisely the same 
conditions, since each kind of flesh is already biased, encumbered by inheri- 
tances. If a certain resemblance between a reptile and mammal dates from 
Permian times, it may be homogenous, like the pentadactyle limb which as such 
has persisted; but if that resemblance has first appeared in the Cretaceous 
period it is Homoplastic, because it was brought about long after the class 
division. To cases within the same order we give the benefit of the doubt more 
readily than if the resemblance concerned members of two orders, and between 
the phyla we rightly seek no connection. However, so strongly is our mode of 
thinking influenced by the principle of descent that, if the same feature happen 
to crop up in more than two orders, we are biased against Homoplasy. 
The readiness with which certain Homoplasies appear in related groups seems 
to be responsible for the confounding of the potentiality of convergent adapta- 
tion with a latent disposition, as if such cases of Homoplasy were a kind of 
temporarily deferred repetition, i.e., after all due to inheritance. This view 
instances certain recurring tooth patterns, which, developing in the embryonic 
teeth, are said not to be due to active adaptation or acquisition but to selection 
of accomplished variations, because it is held inconceivable that use, food, &c., 
should act upon a finished tooth. It is not so very difficult to approach the 
solution of this apparently contradictory problem. Teeth, like feathers, can be 
- influenced long before they are ready by the life experiences of their predecessors. 
A very potent factor in the evolution of Homoplasies is correlation, which is 
sympathy, just as inheritance is reminiscence. The introduction of a single 
new feature may affect the whole organism profoundly, and one serious case of 
Isotely may arouse unsuspected correlations and thus bring ever so many more 
homoplasies in its wake. 
Function is always present in living matter; it is life. It is function which 
not only shapes, but creates the organ or suppresses it, being indeed at bottom a 
kind of reaction upon some stimulus, which stimuli are ultimately all funda- 
mental, elementary forces, therefore few in number. That is a reason why 
Nature seems to have but few resources for meeting given ‘ requirements "—to use 
an everyday expression which really puts the cart before the horse. This 
paucity of resources shows itself in the repetition of the same organs in the most 
different phyla. The eye has been invented dozens of times. Light, a part of 
the environment, has been the first stimulus. The principle remains the same 
in the various eyes; where light found a suitably reacting material a particular 
evolution was set going, often round about, or topsy-turvy, implying amend- 
ments; still, the result was an eye. In advanced cases a scientifically con- 
structed dark chamber with lens, screen, shutters, and other adjustments. The 
detail may be unimportant, since in the various eyes different contrivances are 
resorted to. 
Provided the material is suitable, plastic, amenable to prevailing environ- 
mental or constitutional forces, it makes no difference what part of an organism 
is utilised to supply the requirements of function. You cannot make a silk 
purse out of a sow’s ear, but you can make a purse, and that is the important 
point. The first and most obvious cause is function, which itself may arise as an 
incidental action due to the nature of the material. The oxydising of the blood 
is such a case, and respiratory organs have been made out of whatever parts 
invite osmotic contact of the blood with air or water. It does not matter 
whether respiration is carried on by ecto- or by endodermal epithelium. Thus 
are developed internal gills, or lungs, both of which may be considered as 
referable to pharyngeal pouches; but where the outer skin has become suitably 
