1878.] Morphological Effects of Like Mechanical Conditions. 159 
of Chelonians to agree with ontogenetic history or embryology 
of the individuals of the highest family. This, so far as I have 
been able to make investigations, proves, in a measure, to be the 
case, for it is observed that that portion. of the vertebral axis 
included within the carapace in a young Cistudo has the vertebral 
centra more nearly of the character of the same parts of the low- 
est turtles, which approximate in the development of their centra 
to the normal or usual types of vertebrates with segmented axes. 
The degeneration of the vertebral axis in Czstudo into a mere 
tube, with exceedingly thin walls for the lodgment of the spinal 
cord, may probably be regarded as an instance of Czenogenesis. 
These observed coincidences, it is believed, are neither acci- 
dental nor designed by an active cause external to these organ- 
isms or their cosmic environment. I would rather believe that 
the structures, so far as they have been evolved in parallel or sim- 
ilar ways, are the results of like forces conditioning growth and 
nutrition in definite modes and determinate directions. The 
manner of incidence of the modifying forces being in all cases 
determined by the voluntary actions of the organisms; the actions 
in turn are determined by the degree of intelligence of the animal 
manifesting them. 
The origin of dermal ossifications is to my mind rationally 
explained by supposing the bioplasm of each dermal cell as sen- 
sitive and irritable to rude or violent external impacts, which, oft 
repeated, act as stimuli of growth force, determining certain tracts 
of these cells as the nidus within which osseous particles eventu- 
ally appear as nuclei of the future defensive dermal bony system. 
This happens in the true skin and not in the corneous epiderm, 
which is still retained in more or less rudimentary condition in 
both Chelonians and Armadilloes, though it is not to be forgotten 
that in the toothless old-world Edentata it is the epiderm which 
becomes the defensive covering. This thickening, though not 
depending upon peculiar movements of portions of the body, as 
in the origin of hoofs, horns, etc., of other forms, depends upon 
the motion of the whole body mass, during which the hurtful 
stimulating impacts with the environment take place; so that the 
rationale of the origin of dermal ossifications is finally resolved 
into terms of osteoblasts and animal motion. The likeness of the 
process of the evolution of a defensive osseous or corneous derm, 
as sketched above, to the process of reparation in wounds is very 
