THE GALAPAGOS TORTOISES. 267 
mens commonly said to be 400 years of age. Further observations are needed 
concerning rates of increase. Colnet is to be credited with the statement that 
the nests never contain more than three eggs. Porter says the females without 
exception were full of eggs of which generally from ten to fourteen were hard, 
ready for extrusion. Beck says from ten to twenty eggs are ready for extrusion 
together, while twenty or thirty more were from one half to two thirds the 
normal size. The number of eggs laid by a single female in a season of course 
depends on her size and age; at any rate the evidence indicates she might densely 
populate a given locality, if beyond interference from enemies, in a very few 
years. Neither rate of growth nor abundance of progeny favors the conclusion 
that the tortoises have been on the islands from the very earliest times. Yet in 
estimating the numbers of the tortoises Baur quoting from Reynolds says that 
between October 13, 1832 and August 30, 1833, thirty-one whaleships reported 
at Santa Maria; he adds that if each vessel carried away but 200 it would make 
6,000 from this island alone in less than a year. In alater MS. he states there 
is little doubt that about 10,000,000 tortoises were taken from the islands since 
their discovery. 
The factors of the greatest importance in the differentiation of species 
and varieties are the differences in the altitudes, which in the various islands 
range to 4,000 feet more or less, with the consequent differences in temperature, 
moisture, dryness, food and feeding habits, soil, etc. The variations in rapid- 
ity of growth, sizes attained, increase in numbers and the like are readily traced 
to one or several of these agents. In some of the most superficial characters 
their efficiency is quite perceptible; thus for example in the epiderm, the slough, 
which grows in correspondence with the skin and the bones beneath it. 
Sloughing is a process undergone by reptiles in general. It is part of their 
method of renewing and enlarging the epidermal covering. The new epiderm 
grows under the old one, the slough, between it and the balance of the skin. 
In the majority the discarded epiderm is thrown off at particular seasons. 
On some forms it is retained in one way or another and made to serve useful 
purposes, as protecting the skin or bones, or as claws or spines, and in a few it 
is so greatly modified as to serve as rattles. There are differences among tor- 
toises in regard to the habit. Most of the marine forms slough early in life 
and subsequent sloughs are less noticeable. Soaking in the water aids in slough- 
ing, but on the other hand a dry skin appears to be a more effectual preventive 
of loss and welds the various sloughs together one after another, cementing 
then firmly so that instead of a single thin horny layer of epiderm, of a single 
