But I believe that the effects of
habit are in many cases of subordinate importance to the effects of the
natural selection of what may be called spontaneous variations of
instincts;--that is of variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired--for these could be found only in
the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought
at least to be able to show that gradations of some kind are possible; and
this we certainly can do. I have been surprised to find, making allowance
for the instincts of animals having been but little observed, except in
Europe and North America, and for no instinct being known among extinct
species, how very generally gradations, leading to the most complex
instincts, can be discovered. Changes of instinct may sometimes be
facilitated by the same species having different instincts at different
periods of life, or at different seasons of the year, or when placed under
different circumstances, etc.
Pages:
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399