270 PROFESSOR OWEN ON THE EXTERNAL CHARACTERS 
The long-continued growth and superior size of the Human brain, more especially 
the superior relative size of the cerebral hemispheres and their numerous deep and 
complex convolutions, are associated with psychical powers, compensating for and 
permitting the absence of natural weapons of offence and defence ; they are corelated 
with those modifications of the lower limbs which free the upper ones from any call to 
serve the body in the way of moving or supporting it, and leave them at the command 
of the intellect, for such purposes, in the fabrication of clothing, weapons, &c., as it 
may energize upon according to its measure of activity in the individual. 
In investigating and studying the value and application of the cerebral characters of 
Man in the classification of the Mammalia, I have been led to note the relations of 
equivalent modifications of cerebral structure to the extent of the groups of Mammals 
respectively characterized by such conditions of brain. The Monotremes and Marsupials, 
which offer numerous extreme modifications of the limbs, all agree in possessing a brain 
in which there is no connecting or commissural mass of fibres overarching the lateral 
ventricles of the cerebrum. The surface of this part shows, however, a few symmetrical 
convolutions in Echidna! and Macropus, especially the largest species*; but in the 
majority of Marsupials the hemispheres are smooth. The “‘ corpus callosum,” or great 
commissure, makes its appearance abruptly in the Rats, Shrews, Bats, and Sloths, which 
in general organization and powers are next the ‘‘ loose-brained ” Marsupials or Lyen- 
cephala ; but this commissure is associated with a similarly smooth unconvolute cerebrum, 
and with so small a size of the cerebrum as leaves uncovered the cerebellum and, in 
most, the optic lobes. Only in the largest of the ‘‘ smooth-brained”’ group (Lissencephala) 
are a few simple, symmetrical, cerebral fissures present. I refer to my paper in the 
‘Proceedings of the Linnean Society,’ 1837, for the summary of the characters which 
associate in close contiguity the Cuvierian orders Rodentia, Edentata, Insectivora, and 
Cheiroptera, and indicate their position as next above the Marsupials and below the 
other orders in the ‘ Régne Animal.’ In almost every system of organs, except the 
cerebral, they offer as many and great varieties as do the Lyencephala: the one organ 
that links them together by the uniformity of its developmental grade and structural 
condition is the brain; whence they may be collectively spoken of as ‘* Lissencephala,” 
or the smooth-brained subclass with connected hemispheres. 
The inference as to the respective values of ‘brain,’ ‘ teeth,’ ‘limbs,’ &c., as mam- 
malogical characters is plain: the most constant organ bespeaks the widest group. 
In the remaining and, as I infer, higher orders of Mammalia the cerebrum presents 
a sudden increase of size: if, as in the diminutive Lemurs and platyrrhine Marmosets, 
its surface is smooth, it not only covers the optic lobes, but also more or less of the 
cerebellum. But, as a rule, the grey superficies of the cerebrum is expanded by 
convolutions®. In the platyrrhine and catarrhine Quadrumana the cerebrum becomes 
* Cyclopedia of Anatomy and Physiology, art. “‘ Monotremata,” vol. iii. p. 383. fig. 182. 
* Phil. Trans. 1837, pl. 5. fig. 4. * Even in the Rodent-like Aye-aye. 
