478 COMPARATIVE PHYSIOLOGY. 



The fact that the brain and cord arise from the same germ 

 layer, and up to a certain point are developed almost precisely 

 alike, is full of significance for physiology as well as morphol- 

 ogy. That original deep-lying connection is never lost, though 

 functional differentiation keeps pace with later morphological, 

 differentiation. But even among vertebrates the spinal cord 

 shows a complexity gradually increasing with ascent in the 

 organic series. In the lowest of the fishes or vertebrates {Am- 

 phioxus laneeolatus) the creature possesses a spinal cord only 

 and no brain, so that an opportunity is afforded of witness- 

 ing how an animal deports itself in the absence of those direct- 

 ive functions, dependent on the existence of higher cerebral 

 centers. The Lancelet spends a great part of its life buried in 

 mud or sand on the bottom of the ocean, and its existence is 

 very similar to that of an invertebrate, though, of course, the 

 dependence of parts on each other is somewhat greater. 



Evolution. — According to the general law of habit and in- 

 heritance, we should suppose that at birth each group of ani- 

 mals would manifest those reflex and other functions of the 

 cord which were peculiar to its ancestors. Observation and 

 experiment both show that reflexes, etc., are hereditary ; that 

 they tend to become more and more so with each generation ; 

 and at the same time that habit or exercise is essential for their 

 perfect development. They stand, in fact, in the same relation 

 as instincts, which are closely connected with them. Like the 

 latter, they may be modified by way of increase or diminution 

 and otherwise. To illustrate, it can not be doubted that gallop- 

 ing is the natural gait of horses, as shown by the tendency of 

 even good trotters to "break " or pass into a gallop ; but it is 

 equally well known that famous trotters breed trotters. In 

 other words, an acquired gait becomes organized in the nervous 

 system (especially) of the animal, and is transmitted with more 

 and more fixity and certainty with the lapse of time. But all 

 experience goes to show that walking, running, or any of the 

 movements of animals are, when fully formed as habit-reflexes, 

 dependent for their initiation on the will in most but not all 

 instances, and require for their execution certain combinations 

 of sensory and other afferent impulses, and the integrity of a 

 vast complex of nervous connections in the spinal cord. 



It is well known that one in a period of absent-mindedness 

 will walk into a building to which he was accustomed to go 

 years before, though not of late, showing plainly that volition 



