334 Comparative Physiology. 



" The simpler the condition of any organism, the more sus- 

 ceptible is it of being modified in form and structure by external 

 causes. In the more simple embryonic state also, changes are 

 more easily effected : the germ is hybridised by being furnished 

 with different nutrient matter from another female parent 

 than usual ; the bee is changed from a working neuter to a 

 queen by peculiarities in the cell and food; and, among the 

 lowest groups of plants, there seems reason to believe that the 

 same germ may assume very different forms, according to the 

 circumstances under which it is developed." 



Some have denied the existence of any such thing as stimuli 

 at all, especially as applied to the alimentary materials. It is, 

 they say, the production of the aliment in a proper form, and 

 under the requisite circumstances, that produces activity in the 

 vital functions. When the circulating fluid contains the proper 

 elements, in the requisite condition for absorption, nutrition, 

 &:c., these functions will become active, which would cease to be 

 the case, if the necessary changes were not produced by reaction, 

 &c. The presence of nitrogen in the form of ammonia, and 

 alkaline substances, in the young shoots and leaves of plants, 

 producing the well-known dark-green colour so characteristic of 

 vigour, has been thought to produce a stimulating action on 

 the organs, increasing their activity. It has been sometimes 

 known to take place without vigour of growth following ; and 

 it may be doubtful whether it denotes the proper state of the 

 food for growth merely, or produces a stimulating action. 

 From the excitability prevalent, however, through all organised 

 tissues, their capability of being stimulated to action has been 

 generally inferred. Miiller defines stimulus as a reaction fol- 

 lowing a disturbing cause, something similar to elasticity, in 

 which a power of attraction causes the disturbed particles to 

 communicate the attempt to displace a portion to the whole, 

 and bring into activity a power of restitution, accompanied by 

 elasticity. The power of reaction or restitution, he says, in 

 organised beings, is, however, more uniform than the elasti- 

 city, &c., of inorganic, and arises from that fundamental property 

 resident in them, of counterbalancing disturbances in their 

 composition by a force which, in the healthy state of the body, 

 is much stronger than the disturbing cause. Dutrochet calls 

 excitability a state of susceptibility of excitation. The power 

 of resisting excitation has by others been tei-med a vital property, 

 antagonist of that of the chemical or exciting, which tends to 

 destroy ; and the capability of stimulus would, from the above 

 definition, appear to be a capability of displaying vital actions, 

 or a susceptibility thereto. The quiescent state of the vital 

 principle he terms " a capability of living ;" the simplest or- 

 ganised beings retaining this state longest, and seeming least 



