THE BRAIN IN THE EDENTATA. 375 



Unguiculate groups, but the task of deciding whether some of the smaller mammalian 

 brains belong to the Insectivora, E-odentia, Chiroptera, or possibly Edentata, would tax 

 the ingenuity of the most learned anatomist. However much importance w^e may attach 

 to the value of the evidence presented by the brain, we cannot afford to disregard the 

 testimony of the other systems wpon which, in many cases, we must rely for decisive 

 data for classificatory purposes. 



In attempting to form a just estimate of the value and full significance of our data, we 

 cannot take too wide a view of the field of enquiry. We must strive to clearly appreciate 

 the distinctive features of the mammalian brain aud the factors which are at work in 

 modifying it, before we attempt to estimate the taxonomic value of the features of any 

 given type of brain. The brain indicates, perhaps more thau any other system, the 

 immense superiority of the mammal over all other forms of life. For, in virtue of the 

 possession of such an organ, the mammal is able to very readily adapt itself to more 

 comjilex conditions of existence than is possible among any other class of animals, without 

 the most profound modifications of structure which take extremely long periods of time 

 for their accomplishment. Mammals may, on the other hand, in comparatively very 

 short periods of time and by means of changes which are so slight as not to fundamentally 

 interfere with the distinctly mammalian type of body, adapt themselves to almost any 

 mode of aquatic, terrestrial, or aerial life such as among other vertebrates it takes whole 

 geological epochs to acquire, and then only at the expense of most fundamental modifi- 

 cations of their bodily structure. 



The part of the brain in which this superiority most strikingly manifests itself is the 

 cerebral hemispheres. In all vertebrates the incoming olfactory nerves are inserted into 

 an olfactory bulb, which is connected by a peduncle with the cerebral hemisphere, which 

 at first consists of little else than a small, simple, unditferentiated basal ganglion. At a 

 very early stage in the evolution of the brain, we can recognize the primordial 

 elements of the rhinencephalon making their apjiearauce. Even in the Dij^noi and 

 Amphibia we can recognize a difierentiation of the mesial wall of the primitive liemi- 

 sphere into the paracommissnral body and the forerunner of the hij)pocampus, while on 

 the basal and lateral aspects the undiff'erentiated primordial tuberculum olfactorium and 

 lobus pyi'iformis may be localized. In tlie Sauropsida the primordial hippocampus and 

 pyriform lobe advance a further stage in the process of differentiation, although they ai'e 

 still far removed from the final stage of elaboration which we find in even the lowest 

 mammal. 



It is only in the Mammalia that the pyriform cortex becomes fully differentiated and 

 the hipjiocampus assumes its typical characters. Now, for the first time, the margin of 

 the liippocampal formation becomes strangely modified to form the fascia dentata, and 

 in association with this the rt^gion undergoes a ])eculiar mechanical involution which 

 gives rise to the hippocampal fissure. These two regions — the pyriform lobe and the 

 hippocampus — not only reach the acme of their elaboration within the Mammalia, but 

 they increase considerably in size, so that in the early mammal they form a large and 

 complicated mechanism in which olfactory impulses are received and possibly blend with 

 the impulses of other sense-organs. 



51* 



