14 Animal Life and Intelligence. 



difference, are due to heredity with variation. But this 

 merely names the facts. How the special reproductive 

 cells have acquired the secret of developing along special 

 lines, and reproducing, with a margin of variability, the 

 likeness of the organisms which produced them, is a matter 

 concerning which we can at present only make more or 

 less plausible guesses. 



Scarcely less wonderful is the power which separated 

 bits of certain organisms, such as the green freshwater 

 hydra of our ponds, possess of growing up into the com- 

 plete organism. Cut a hydra into half a dozen fragments, 

 and each fragment will become a perfect hydra. Repro- 

 duction of this kind is said to be asexual. 



We shall have, in later chapters, to discuss more fully 

 some of the phenomena of reproduction and heredity. 

 For the present, it is sufficient to say that animals repro- 

 duce their kind by the detachment of a portion of the 

 substance of their own bodies, which portion, in the case 

 of the higher animals, undergoes a series of successive 

 developmental changes constituting its life-history, the 

 special nature of which is determined by inheritance, and 

 the result of which is a new organism in all essential 

 respects similar to the parent or parents. 



11. Animals are living organisms, and "not r<'<ictal>les." 

 The first part of this final statement merely sums up the 

 characteristics of living animals which have gone before. 

 But the latter part introduces us to the fact that there are 

 other living organisms than those we call animals, namely, 

 those which belong to the vegetable kingdom. 



It might, at first sight, be thought a very easy matter 

 to distinguish between animals and plants. There is no 

 chance, for example, of mistaking to which kingdom an 

 oak tree or a lion, a cabbage or a butterfly, belongs. But 

 when we come down to the simpler organisms, those whose 

 bodies are constituted by a single cell, the matter is by no 

 means so easy. There are, indeed, lowly creatures which 

 are hovering on the boundary-line between the two 

 kingdoms. We need not discuss the nature of these 



