IV SHIFTS FOR A LIVING 51 



been left in a pool by the retiring tide. How striking 

 again are the ways of the land-crab Birgus (see Fig. 10), 

 which goes far up the mountains and even climbs the 

 coco-palm for the nuts ! It is hardly less remarkable 

 that it is able to make a hole in the shell. It feeds on 

 the pulp, uses the fibre of the nut in its burrow, and 

 sometimes carries part of the shell about with it as a 

 protective covering for its abdomen. Every year, how- 

 ever, Birgus returns to the sea-shore to breed, and its 

 marine larvae well illustrate the general conclusion that 

 the young forms of a species occur in the ancestral 

 habitat. The turtles that lay their eggs ashore illus- 

 trate the same law though the movement is in the oppo- 

 site direction. 



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, tape- 

 worms, 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 per- 

 manent, external and internal, very degenerate, and 

 very slightly affected parasites. Sometimes the adults 

 are parasitic while the young are free-living, 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 devoured 

 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 in- 

 sects 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 advan- 

 tages gained by a thoroughgoing parasite are great- 

 safety, warmth, abundant food, in short, " complete 

 material well-being." But there is another aspect of 

 the case. Parasitism tends to be followed by degenera- 

 tion of appendages, food-canal, sense-organs, nervous 

 system, and other structures, the possession and use of 



