156 BIOLOGY: GENERAL AND MEDICAL 



made out in animals lower than the arthropods, with 

 the single exception of the leeches. 



The increasing complexity of the central nervous 

 system depends in large measure upon the continually 

 increasing importance of the organs of special sense, 

 which improve in quality and number and require 

 means by which the impulses they receive may be trans- 

 mitted to and from the common utilizing and governing 

 centre. 



It might seem as though the tactile sense, that through 

 which the organism is able to recognize the existence 

 of objects external to itself ought to be the first of the 

 special senses, yet it must not be forgotten that the 

 most primitive forms of life are not only subject to the 

 injurious or beneficial effects of contact with objects, 

 but are at the mercy of every force known to the physi- 

 cist so that inability to avoid the harmful and avail 

 themselves of the useful must result in death. Vast 

 numbers of organisms must die every moment because 

 of their inability to discriminate, and it must be only 

 by the force of the numbers developed when conditions 

 happen to be favorable that such are able to persist at 

 all. 



In the absence of visible means of receiving impres- 

 sions from the external world, the behavior of these 

 primitive forms is said to be controlled by forces already 

 described as tropisms. 



Looking in retrospect over the gradations of life 

 between the highest and lowest of living things, both 

 animal and vegetable, one is struck by the fact that 

 accident has much to do with success or failure in sur- 

 viving in the midst of what appear to be antagonistic 

 influences. At first thought it might seem as though 

 the necessity for sensory organs for the appreciation of 

 external conditions, and by virtue of which a suitable 

 environment might be sought and an unsuitable one 

 avoided, enemies eluded and food captured ought to 

 be indispensable to successful existence, yet the far 



