668 ANIMAL PHYSIOLOGY. 
den to an already weakened organism. Nervous discharges 
tend more and more to be slow, weak, and to take the lines 
fixed by long usage; hence, perhaps, that undue conservation 
of mind common to the old; that lack of enterprise, which is 
strengthened by the consciousness of inability, physical and 
mental, for the strain of néw undertakings. Hence also the 
natural failure of acquiring power and the memory. The 
judgment, dependent as it is on accumulating experience, im- 
proves. With extreme old age there is a reversion to the 
infantile condition, marked by irritability of tissues, weak- 
ness, etc. 
The laws of habit and rhythm are illustrated abundantly 
in the subjects we have been considering. Rhythm seems to 
be a sort of key-note to the interpretation of the universe; but 
since we have frequently referred to this subject throughout 
the volume, it will not be further dwelt upon now. 
Comparative.— All mammals have their periods of rapid 
growth, slower decay, and death. Their growth is usually 
more rapid than man’s, and as their whole lives are shorter, 
with few exceptions, their rate of decay is faster. There are 
great differences between various mammals in their degree of 
development at birth. Among some (the marsupials) they 
separate from the mother internally, to become attached to the 
nipples externally when very imperfectly developed. Though 
puppies, kittens, and other members of the groups to which 
they belong (cornivora) are born with the eyes unopened, no 
mammal is so helpless as the human infant when ushered into 
the world. Most animals learn the use of their muscles, and to 
provide for themselves in a very short period. Slowness of 
development is, however, even among the lower animals, fre- 
quently associated with the attainment of an ultimately higher 
functional status, and the precocious child should be the object 
of some anxiety. It may develop into a prodigy of talent, rise 
little above mediocrity, or become the subject of some serious 
or fatal form of disease. 
It is important to recognize that sexual maturity, in the 
sense of ability to-produce ripe ova and spermatozoa, does not 
correspond with the full development of the animal; so that it 
may be as unscientific to breed together animals that are very 
young as those that are decaying from age. Especially is it 
undesirable to mate two very young or very old animals. Such 
a principle applies, of course, also to man. 
Death.—If the continuance of life is dependent on the cease- 
