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MAN IN THE  MODERN WORLD

shown that the smaller the island, and therefore the smaller its lizard
population, the more different this has become from the mainland
type from which it was originally derived (see Table).

DIFFERENTIATION IN ISLAND LIZARDS

area

(arbitrary      0-6     6-12       12-18    18-24      24~3°    30-36    depth

(m)

units) <o-5
	*4
	
	at
	
	
	

0-5-1
	I
	I
	
	
	
	

i-5
	I, 2
	I, 2
	2, 2£, 2& 3
	3
	
	4

5-10
	I
	
	
	
	
	

IO-IOO
	o
	
	2
	
	
	

IOO-IOOO
	
	
	O
	
	
	

Table showing the influence of time and of size of population on the differentia-
tion of island lizards from the mainland form. The depth is the maximum depth
of water between the island and the mainland; as the islands have been formed
by subsidence, the depth gives a measure of the time since isolation occurred. The
area represents the area of the island, which is a measure of the population. The
figures 1-4 in the chequer-board represent degrees of difference of the island forms
from the mainland form. It will be seen that on the whole the longer the time of
isolation and the smaller the size of the population, the greater is the degree of
divergence. (Reproduced by kind permission of the publishers of J. S, Huxley's
Evolution : the Modern Synthesis, Messrs. Allen & Unwin.)
The other chief method by which new species are formed is through
genetic isolation. This happens when a new form, wholly or partly
infertile when crossed with its parent, is produced by some genetic
accident—by means of the reduplication of whole chromosome sets,
196