394 
On the Sesamoids of the Knee- Joint 
A study of either Carnivores or Primates indicates what a long course of evolu- 
tion is involved in the loss of a sesamoidal system, and what a wide range of 
changing forms usually intervenes. The persistency of the same type of sesamoidal 
system for generations in the same species, and the occurrence of atavisms, such as 
lunulie in man, form strong evidence that the minor sesamoids of the knee-joint 
are as hereditary in character as the patella. 
To assert that a sesamoid can be produced by " intensive stress " when and 
where it is required is to assert in its baldest and most futile form the inheritance 
of acquired characters. The most futile form because a parafibula, say, is not a 
merely formless mass, it is an organised link for a system of muscles, usually of 
a very elaborate kind. And if it could be acquired by " intensive stress" when and 
where needed, there would seem no reason to despair of any species producing a 
new bone of the skeleton, when and where needed. It is conceivable that a mutation 
could produce such a bone by an anomalous disturbance of the germ cells — but it 
would certainly not be due to " intensive stress" at the required point, and it would 
only be by the rarest chance that it would be " when and where needed." The 
whole mechanism of heredity is disregarded by such anatomical suggestions. 
In forming the .scheme of our diagram we wish to emphasise the fact that while 
there are factors of acquisition and suppression at work, we yet consider the primary 
factor to be heredity — the form of the sesamoidal system is inherited as the form 
of the teeth are inherited, and the change of form will in each case be a lengthy 
evolutionary process*. 
In our diagram we have purposely refrained from providing any descent links, 
partly because there is such an immense amount of work still to be done on the 
sesamoid systems especially of reptilian and avian forms, and partly because to do 
so would be sure to give rise to the criticism that we had supposed one illustrative 
type to have sprung directly from a second illustrative type ; whereas in fact both 
illustrative types may have been only in cunahula at the time of the link. The 
link lies between the sesamoidal systems, not necessarily between their present 
representatives. 
We have really fo^^r origins to study : 
(a) That of the patella ; it may have arisen from the tibial crest or from an 
antero-sagittal humla, or perhaps have had both origins. At present we have not 
come across any evidence of a 'tibial crest in the Reptilia except in the Dinosaurs. 
(b) That (or better, perhaps, those) of the lunulae. We have no suggestion to 
make here. It needs a far greater study of the knee-joint of both Amphibia and 
Reptilia than we have been able to make. 
* That in an early period of evolution the germ cells of various allied species were not so highly 
specialised and differentiated as in a later period, is a possibility that we have not seen emphasised, and 
which would, perhaps, render by hybridisation the earlier evolutionary processes quicker. Even now we 
have very vague ideas of the limits of possible hybridisation. How far are the Primates inter se fertile ? 
The fox and dog are natural enemies, but the cross is a feasible one, and will provide a second generation. 
Much of importance for evolution might arise by crossing — if needful by artificial impregnation — forms 
apparently fairly diverse. Divergence of habits and temperament and somatic differences may have more 
to do with the problem of infertility than any factor of the germ cells themselves. 
