38 THE WONDER OF LIFE 



the herbage and ' sniffs furiously ' as it goes ; it often calls 

 to its mate ; it is at no pains to keep quiet. Nor need it, 

 for although a few enemies manage to eat it, the spines are 

 in most cases quite effective prevention. Moreover, it 

 can give rise when irritated to a most horrible stench. The 

 porcupine is another good instance of a self-advertiser, 

 and so is the crab-eating mungoose (Mungos cancrivora). 



In a few cases we have some definite knowledge in regard 

 to the actual process of adaptive colour- change. The spotted 

 salamander (Salamandra maculosa), with its conspicuous 

 livery of bright yellow and dark brown, is a case in point. 

 It is well known to become almost black when the soil of 

 its vivarium is dark and relatively dry. Two things 

 happen : the yellow areas become gradually smaller, re- 

 treating towards the centre till they disappear ; and the 

 dark areas become darker. Experiments following the 

 ordinary method of exclusion are very instructive, e.g., 

 using a black-paper ground with the normal humidity. 

 The shrinkage of the yellow spots is induced by the colour 

 of the ground, while the darkening is brought on by 

 increasing drought. An experimenter, Alois Gaisch, 

 relates that he put a salamander into a vivarium with 

 black peaty soil (which remained moist), and found it 

 almost unrecognizable after three months. The yellow 

 spots had shrunk, there were many black dots about their 

 margins, and microscopic examination showed that the 

 black pigment had abundantly invaded the yellow areas. 

 Two other salamanders put in about the same time showed 

 no change of colour, which seems to show that there are 

 differences in individual susceptibility. If that be so, and 

 if it had not to do with age or the like, we have an illustra- 

 tion of how a selective process might work. For if it were 



