194 THE WONDER OF LIFE 



compared to the composition of forces — this organic 

 compromise — ^because so much depends on the physiological 

 state of the creature at the time being. 



Changes of Reaction dependent on Internal Condi- 

 tions. — Modern experimenting has made clear that a crea- 

 ture's activity at a given time is, in part, dependent on 

 the general physiological state of its body, apart from the 

 activity of the central nervous system. And the physiolog- 

 ical state of the body alters with functioning and environ- 

 ment. A well-known instance observed by Loeb is very 

 striking. The caterpillars of Porthesia chrysorrhcea, which 

 emerge from hibernation in spring, have a very pronotmced 

 attraction to hght (positive phototropism). But when they 

 have eaten, this disappears entirely, and does not reappear. 

 The physiological state of the body has been thoroughly 

 changed, and the behaviour likewise. Loeb also notes that 

 when the male and female ants are approaching sexual 

 maturity, they exhibit an intense and increasing attraction 

 to the Ught, which the workers do not show. The physio- 

 logical state of the body has been altered by the onset of 

 reproductive maturity, and the behaviour is correspondingly 

 changed. 



After an animal has reacted many times in rapid succes- 

 sion to the same stimulus, it ceases to do so. Some active 

 substance in the sensory cells, or in the nerve cells, or in the 

 muscles, has been used up for the time being. The weak 

 reactions before the substance was quite used up and 

 before it has been properly restored have been put down 

 to the creature remembering that it had been fooled these 

 many times. But it is almost certain that this is quite 

 wrong, and that there is no memory involved at all. 

 Cellular memory begins when there is some more or less 



