78 HOUSE, GARDEN, AND FIELD 



seas, which abound in animal life beyond any other part 

 of the earth's surface, favours early dispersal. Crowding 

 brings competition and multiplies enemies, so that the 

 risks to be faced by any species are intensified. Hence 

 a greater number of embryos have to be produced in order 

 to keep up the number of the species. These numerous 

 embryos cannot be subsisted directly or indirectly by the 

 parent, and must therefore be turned out at once to shift 

 for themselves. Their first necessity is dispersal, and 

 hence the embryos of littoral animals are almost of 

 necessity minute, provided with temporary locomotive 

 organs, and quite unlike their parents in form as well as 

 in size. Barnacles, like the majority of shore- animals, 

 are obliged to produce vast numbers of eggs on account 

 of the risks which are to be run. The eggs are minute 

 and ill-furnished with yolk. The issuing embryos are 

 altogether unlike their parents, and adapted to a life of 

 free movement. 



The rock-barnacle is a good example of development 

 with transformation of the kind most usual among 

 animals. The frog and the insect, though often chosen 

 as examples, because they are familiar to dwellers in 

 inland places, are examples of an unusual kind. They 

 do not migrate first, and then settle down to feed, but 

 acquire by a late transformation the power of migrating. 

 In insects the transformation is often nearly the last event 

 in the life-history. 



XV. RATS AND MICE. 



If you were suddenly called upon to define a rat or a 

 mouse, you would come off pretty well if you were to say 

 that it was a small quadruped with short hair, clawed 

 feet and a long tail. The zoologist would not consent, 

 however, to put into one family, nor even into one order, 

 all the animals which exhibit these characters. He would 

 point out that some of them are sharply distinguished 



