ANIMAL BIOLOGY. [Chap. I. 



frog. S@, too, in the case of the chick. We know that the egg 

 we eat for breakfast, if it had been placed for three weeks or so 

 under a hen, would have developed into a little chicken. But 

 not only the frog and the chick, but the dog, the worm, the 

 butterfly, the star-fish, are one and all developed from an egg 

 which is at first just a little speck of living matter. So that we 

 may say that living animals, during their growth, pass from a 

 comparatively simple condition to a comparatively complex con- 

 dition by a process of change which we call development. 



Now it is clear that, since we have no knowledge of dead 

 matter springing into existence as living matter, life on the 

 earth would soon cease if there were not something more than 

 growth, development, decay, and death. Since death is the 

 heritage of living things, we have the necessity for reproduc- 

 tion. This process is essentially the detachment of a part of the 

 parent organism, which part itself, in turn, develops, reproduces, 

 decays, and dies. In the higher animals reproduction becomes 

 possible when growth and development are ceasing. The excess 

 of repair over waste is seen, not in growth, but in the periodic 

 detachment of a portion of the organism to continue its kind. 



Another fact must now be introduced since it is one that is 

 eminently characteristic of living things. In any animal the 

 line along which the series of changes (growth, development, 

 etc.) takes place is not indeterminate, but is determined by 

 inheritance. Every mammal, e.g., begins life as a minute speck 

 of an egg. There is often nothing about the egg to tell us to 

 what particular animal it will give rise. And yet this is already 

 quite determined by inheritance. So that we may say, sum- 

 ming up so far, that a living animal is a centre of continual 

 waste and repair ; it undergoes a series of successive develop- 

 mental changes constituting its life-history, the special nature 

 of which is determined by inheritance ; it reproduces its kind 

 by the detachment of a portion of its own substance. 



And what is that substance ? The essential constituent of a 

 living animal is protein, composed of carbon, hydrogen, oxygen, 

 and nitrogen, with a little phosphorus and sulphur. This, with 

 much water, forms the chief constituent of protoplasm. At pre- 



