Karl Pearson and Adelaide (i. Davin 
395 
(c) That of cyamella and lateral /a6ei^a. We are fairly convinced that this lies 
in the break-up of the parafibula and that the parafibula was the result of the 
separation of the fibular crest with its attached muscular system. The parafibula 
exists in certain reptiles. The discovery of a reptilian fibular crest would be of the 
greatest importance. Of course there may be again a double origin corresponding 
to the parafibula of the Reptiles and the fibular crest of the Monotremes. 
{d) That of the appearance at a relatively late stage of evolution (and the 
early disappearance) of the mesial fahella. We have no real light to throw on this 
point ; it needs a very special study. The fact that the mesial fahella is never 
specific without the lateral, while the lateral is in early as well as in late evolu- 
tionary stages specific without the mesial, suggests that the explanation may be 
cytological. At the same time we have to remember that while the lateral head of 
gastrocnemius had originally a fibular or at least a parafibular attachment, the 
evolutionary history of the inner head seems to point to no such marked change in 
its attachments*. 
The object of this memoir has been to suggest problems to those better equipped 
for studying them than the present authors, rather than to present solutions. 
They believe that the least important elements of the skeletal form are each able 
to contribute important suggestions for the general attack on the enigma of evolu- 
tion and that all vestiges are of special value in the assault. They might even 
be content with the outcome of this paper if it led anatomical text-book writers 
in the future to drop once for all the Galen tradition and assert on the contrary 
that it is needful that all sesamoids should be discussed. 
As final conclusion may we not cite the words used by Pittard in 1849 as 
appropriate to our own views : " But after all, taking into consideration all the facts 
related above, and many others that have presented themselves to us in the course 
of this enquiry, we cannot but believe that some higher law than that of adapta- 
tion concurs in determining the presence, if not the size, of even these little 
bones f." 
We have to acknowledge a grant from the Dixon Fund for Scientific Investiga- 
tion allotted by the Senate of the University of London towards the cost of the 
illustrations of this memoir. 
We wish further to thank Sir Arthur Keith for the invariable kindness he has 
shown us in freely placing the immense stores of valuable material in his charge at 
our disposal. For aid in a great variety of enquiries we have to thank very cordially 
Dr E. C. Derry of Cairo, Mr W. R. Sherrin of the South London Botanical Institute, 
Dr W. C. Mackenzie of the Australian Institute of Anatomical Research, Melbourne, 
and Dr Julia Bell of the Galton Laboratory. We have also to acknowledge the very 
helpful assistance of Miss I. McLearn, especially in the drawings of the knee-joints 
of birds. 
* It may have in certain cases an insertion to the tibial crest : see the second footnote on p. 392. 
t Article on Sesamoid Bones, Todd's Cyclopaedia of Anatomy and Physiology, p. 543. 
