CHAPTER XI 

 THE SENSE-ORGANS 



'* Die wunderbare und wirklich iiberraschende 

 Ahnlichkeit in der inneren ' Organisation, in den 

 anatomischen Structurverhaltnissen, und die noch- 

 merkwiirdigere Ubereinstimmnng in der embryonalen 

 Entwickelung bei alien Thieren, welche zu einem und 

 demselben Typus, z B., zu dem Zweige der Wirbel- 

 thiere, gehoren, erklart sich in der einfachsten Weise 

 durch die Annahme einer gemeinsamen Abstammung 

 derselben von einer einzigen Stammform. Entschliesst 

 man sich nicht zu dieser Annahme so bleibt jene durch- 

 griefende Ubereinstimmung der verschiedensten Wir- 

 belthiere im inneren Bau und in der Entwickelungs- 

 weise vollkommen unerklarlich." 



ERNST HAECKEL, Schopfungsgeschichte, Kap. III. 



IT will be remembered that the nervous system is primarily 

 external, developed in response to stimuli from without, and 

 that, as this system becomes more specialized, and hence of 

 greater importance to the organism, it withdraws in great part 

 into the interior, leaving upon the surface a set of sense-organs, 

 capable of receiving the impressions and transmitting them to 

 the central organ. Taking into consideration the intimate con- 

 nection between these two portions, external and internal, 

 it might be supposed that the enormous development in size 

 and complexity shown by the brain would be the result of a 

 corresponding degree of differentiation of the external parts, 

 yet such is by no means the case. The sense-organs are early 

 brought to a high state of efficiency and develop but little 

 during the entire vertebrate history. The eye of the fish is 

 almost as good an optical instrument as is that of the mam- 

 mal, and, save for a few external parts, is as complex; the 

 sense of hearing, although not as early in development as the 

 eye, is yet very acute in reptiles, and, perhaps, in amphibians, 

 and gains in birds and mammals very little except, perhaps, 



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