62 
ISLAND LIFE. 
[part I. 
from the influx of new or more highly organised animals, we may 
there expect to find the remains of groups which have elsewhere 
become extinct. In islands which have been long separated 
from their parent continents these conditions are exactly fulfilled, 
and it is in such that we find the most striking examples of the 
preservation of fragments of primeval groups of animals, often 
widely separated from each other, owing to their having been 
preserved at remote portions of the area of the once widespread 
parental group. There are many other ways in which portions 
of dying out groups may be saved. Nocturnal or subterranean 
modes of life may save a species from enemies or competitors, 
and many of the ancient types still existing have such habits. 
The dense gloom of equatorial forests also affords means of 
concealment and protection, and we sometimes find in such 
localities a few remnants of low types in the midst of a general 
assemblage of higher forms. Some of the most ancient types 
now living inhabit caves, like the Proteus, or bury themselves 
in mud like the Lepidosiren, or in sand like the Amphioxus, 
the last being the most ancient of all vertebrates ; while the 
Galeopithecus and Tarsius of the Malay islands and the potto 
of West Africa, survive amid the higher mammalia of the 
Asiatic and African continents owing to their nocturnal habits 
and concealment in the densest forests. 
The Rise and Decay of Species and Genera . — The preceding 
sketch of the mode in which species and genera have arisen, 
have come to maturity, and then decay, leads us to some very 
important conclusions as to the mode of distribution of animals. 
When a species or a genus is increasing and spreading, it 
necessarily occupies a continuous area which gets larger and 
larger till it reaches a maximum ; and we accordingly find that 
almost all extensive groups are thus continuous. When decay 
commences, and the group, ceasing to be in harmony with its 
environment, is encroached upon by other forms, the continuity 
may frequently be broken. Sometimes the outlying species 
may be the first to become extinct, and the group may simply 
diminish in area while keeping a compact central mass; but 
more often the process of extinction will be very irregular, and 
may even divide the group into two or more disconnected 
