CHAP. IV Shifts for a Living 47 



2. Concealment. — A change of habitat and mode of life 

 is often as significant for animals as it is for men. It is 

 easy to understand how mammals which passed from 

 terrestrial to more or less aquatic life, for instance beaver 

 and polar bear, seals, and perhaps whales, would enjoy 

 a period of relative immunity after the awkward time 

 of transition was over. So, too, many must have passed 

 from the battlefield of the sea -shore to relative peace 

 on land or in the deep-sea. In a change from open air 

 to underground life, illustrated for instance in the mole, 

 many animals have sought and found safety, and the 

 change seems even now in progress, as in the New 

 Zealand parrot Stringops, which, having lost the power 

 of flight, has taken to burrowing. Similarly the power 

 of flight must have helped insects, some ancient saurians, 

 and birds out of many a scrape, though it cannot be 

 doubted that this transition, and also that from diurnal to 

 nocturnal habits, often brought only a temporary relief. 



3. Parasitism. — From the simple Protozoa up to the 

 beginning of the backboned series, we find illustrations of 

 animals which have taken to a thievish existence as unbidden 

 guests in or on other organisms. Flukes, tapeworms, and 

 some other " worms," many crustaceans, insects, and mites, 

 are the most notable. Few animals are free from some kind 

 of parasite. There are various grades of parasitism ; there 

 are temporary and permanent, external and internal, very 

 degenerate, and very slightly affected parasites. Some- 

 times the adults are parasitic while the young are free-liv- 

 ing, sometimes the reverse is true ; sometimes the parasite 

 completes its life in one host, often it reaches maturity only 

 after the host in which its youth has been passed is de- 

 voured by another. In many cases the habit was probably 

 begun by the females, which seek shelter during the period 

 of egg-laying; in not a few crustaceans and insects the 

 females alone are parasitic. Most often, in all probability, 

 hunger and the search for shelter led to the establishment 

 of the thievish habit. Now, the advantages gained by a 

 thoroughgoing parasite are great — safety, warmth, abund- 

 ant food, in short, " complete material well-being." But 



