336
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THE _BIOLOGY OF FISHES
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species that at present occupy the fresh waters of Britai—n;fbr
example, have arrived since the Glacial Age. Some of them
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are relicts of a more northern fauna, which have become-so
specialised and differentiated in separate waters that ‘eaCh'
lake contains its own peculiar race or species. We must
conclude from this, as from variations in general, that the
environment cannot be left oùt of account.
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Taking this into consideration, the simplest explanati0n
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of instincts is, that intelligence was present at their
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and that improvements came from the aequirement or
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learning of the co—ordinations at increasingly early
under the same environment.
The modications in the“
nervous system have arisen, in fact, in the same way as_the
modications in the external characters.
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1 , Even the inheritance of instincts acquired or learnt by
the}individual during his lifetime, is a possibility that
cannot ”be exclud‘ec‘l.
One or
or half adozen generations,‘Ïf:ÏÊ-Ÿj
do not provide a fair test. We have to take thetime £aCto’ï
into account;
the case of the above—mentioned she‘S
at least ten thousand years havepassed since they became”
isolated in British waters.
And we know that among human
beings
:haVe ; been handed down from parent to
o3pring until after a few, generations they seem1
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inherited
Apart from the inhetanœ of'thc‘f
same‘ environment; it is possible that the regulation of the
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system in particular Waysï=may— have:'a
effect oi_1;the_germplasm,, '_;f
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Certampathsor connections=beg{tweeng different
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