The Making of Species 
of development along disadvantageous lines, or 
to disadvantageous degrees. The statistical 
studies of variation have made known numerous 
cases where the slight, as yet non-significant (in 
a life-and-death struggle) variation in pattern of 
insects, in dimensions of parts, in relative pro- 
portions of superficial non-active areas, are not 
fortuitous, that is, do not occur scattered evenly 
about a mean or mode according to the law of 
error, but show an obvious and consistent tendency 
to occur along certain lines, to accumulate in 
certain directions.” 
It seems to us that the only proper attitude to 
adopt in the present state of our knowledge is, 
not to call in to our aid an unknown growth- 
force, but simply to say that there is evidence 
to show that variations frequently occur along 
certain definite lines only. 
Darwin’s second assumption was that there is 
no limit to which variations may be accumulated 
in any direction; that by adding one minute 
variation to another through countless generations 
new species, new genera, new families may arise. 
This assumption, if applied to continuous or 
fluctuating variations, seems opposed to facts. 
All the evidence available goes to show that 
there is a definite limit to which minute variations 
can be accumulated in any given direction. No 
one has succeeded in breeding a dog as large as 
a horse, or a pigeon with a beak as long as that 
68 
