188 OUTLINES OF EVOLUTIONARY BIOLOGY 



existence of the first of these means of communication. So also 

 does the modern theory of hormones, chemical suhstances which 

 are believed to exercise a most important controlling influence 

 over parts of the body that may be far remote from the 

 place where they themselves are secreted, 1 and the aid of 

 which has also been invoked to account for the phenomena 

 of heredity. Others, again, as we have seen, have called in the 

 aid of the second means of communication to account for the 

 transmission of stimuli from somatic to germ cells, but their 

 view is hardly supported by what we know of the arrangement of 

 the nervous system throughout the animal kingdom, and the 

 difficulty of accepting it is still greater in the case of plants, in 

 which no definite nervous system is developed. The third method 

 of communication, however, seems open to no objection, and, 

 whether it may be supplemented by the others or not, seems 

 amply sufficient to account for the facts. 



The faculty of receiving and responding to stimuli of various 

 kinds is one of the most characteristic features of living proto- 

 plasm. In the higher animals this function is more or less 

 concentrated in definite sense cells, each of which is adapted for the 

 reception of stimuli of one particular kind. The visual sense cells 

 are adapted for the reception of stimuli from the light-vibrations 

 of the ether, the auditory sense cells are stimulated by vibrations of 

 the air or water in which the animal lives, the olfactory sense cells 

 are stimulated by the chemical action of material particles, and 

 the tactile cells by the mechanical stimuli of contact and pressure. 

 In such an organism as the unicellular Amreba, on the other 

 hand, all parts of the superficial protoplasm are probably sensitive 

 to stimuli and no special organs of sense are developed. We may 

 well suppose, therefore, that the undifferentiated germ cells of the 

 multicellular animals and plants are sensitive to stimuli received 

 from the somatic cells, though it is impossible in the present state 

 of our knowledge to determine the nature of these. 



It is not difficult to demonstrate that one cell may actually be 

 stimulated by another without the existence of any protoplasmic 

 connection between the two, as, for example, in the mutual 

 attraction of male and female gametes. Such stimulation, it is 

 true, is usually attributed, mainly at any rate, to the secretion 

 of some specific chemical substance which diffuses into the sur- 

 rounding water, and classed accordingly under the head of 



i Vide p. 126. 



