78 Transactions Texas Academy of Science. — 1906. 
of the insufficiency of the excretion process, therefore the limit of 
life is a matter of excretion. 
Excretion is a removal from the organism of harmful substances. 
That is a special excretory organ that excretes not only its own 
waste products but also materials collected from other organs. Just 
so long as a perfect excretion process is maintained the organism can 
survive. But with advance of age the process becomes impaired, in 
part because the extent of the excreting surfaces do not keep pace 
with the increase in mass. It is only necessary here, as an illustra- 
tion, to indicate one series of phenomena, namely, the increasing depo- 
sition of pigments in the various tissues, particularly the outer skin 
and the nerve cells. Eisig 5 was the first morphologist to argue that 
pigments are excretory masses, and that they increase in amount 
with age. Such pigments present us with the most perceptible proof 
of the accumulation of waste within the body. The organism strives 
its utmost to accomplish perfect excretion. Most of the tissues have 
some power in this direction. As the individual increases in size 
it may develop a succession of more perfect excretory organs; thus 
in many groups of animals larval kidneys arise to be later replaced 
by more adequate permanent ones ; there may even be a succession 
of three kidney systems as in certain Vertebrates and Annelids. In- 
deed it is a significant fact that when there is an exquisite larval 
stage unprovided with specific excretory organs, that larval body 
'dies; this is evidenced by Echinoderms, Nemertines, Ectoprocta, and 
Enteropneusta. We may state that the organism replaces one excre- 
tory system by another just so long as it maintains active growth 
change, but that sooner or later the definitive organs become exhausted 
by the demands of the organisms, waste matter comes to be retained, 
therefore self -poisoning, then death result. 
Now that part which dies is the most specialized part of the organ- 
ism; in the case of the Protozoan it is the residuum, in the case of 
the Metazoan, the soma. We mean by “specialization” that a par- 
ticular function is carried out at the expense of others; and the sig- 
nificance of ontogenetic specialization, or what is called differentiation, 
is that generalized parts become narrowed in function by the succes- 
sive change or even loss of tendencies that they had at the start. 
Specialization need not imply greater complexity of structure or 
process, but rather increasing definite application of a particular part 
and process, entailing the elaboration or secretion of specific sub- 
stances. In other words, the specialized portion dies because it has 
come to perform secretion at the expense of excretion. Thus Minot 6 
5 Monographie der Capitelliden, Fauna und Flora des Golfes von Neapel, 16. 
1897. 
6 On Heredity and Rejuvenation, 1896, American Naturalist. 
