496 ANIMAL PHYSIOLOGY. 
shows a complexity gradually increasing with ascent in the 
organic series. In the lowest of the fishes or vertebrates (Am- 
phioxus lanceolatus) 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 
was not momentarily required for the act of walking and all else 
that is involved in the above behavior. It suggests that certain 
nervous and muscular connections have been formed, function- 
ally at least. Plainly, then, we should not expect each indi- 
vidual man’s spinal cord to be the same, but that the series of 
mechanisms of which every spinal cord is made up should differ 
with experience; and if this holds for individuals, how much 
