ANIMAL BIOLOGY. [Chap. I. 



stimulus of light. But while a large proportion of the activity 

 of the organism is due to its sensitiveness, a certain proportion, 

 which increases as we rise in the scale of organisation, is spon- 

 taneous, for the animal contains the springs of action within 

 itself. It will be well, therefore, for us to replace the particular 

 statement, that animals move about, by the more general, com- 

 prehensive, and exact statement, that they exhibit certain 

 activities, prompted from within or called forth by surrounding 

 conditions. So that we may now finally state the character- 

 istics of living animals as follows : A living animal consists of 

 - an aggregate of protoplasmic cells, together with certain cell- 

 products ; it is a centre of waste and repair ; it undergoes 'a 

 series of developmental changes constituting its life-history, 

 the special nature of which is determined by inheritance ; it 

 exhibits certain activities by which it maintains its relation to 

 surrounding conditions ; it reproduces its kind by the detach- 

 \ ment of a portion of its own substance. 



> This definition or description applies to all the higher 

 animals; but we shall learn hereafter that there are many of 

 the lower .animals that are not cell-aggregates, but are each of 

 them constituted by a single cell. These unicellular animals 

 are called protozoa, or cytozoa, while the multicellular organisms, 

 in which, as we shall see, different cells have different modes of 

 activity, are called metazoa or histozoa. 



Let us now consider the essential points in which one of these 

 higher animals the animals with which we are ordinarily 

 acquainted differs from one of the higher plants the plants 

 we see around us. But first we may note the points of resem- 

 blance. They run almost through the whole description given 

 above. Both animal and plant are cellular and protoplasmic ; 

 both grow by intus-susception ; both are centres of waste and 

 repair ; of both there is a developmental life-history ; in both is 

 reproduction similar in principle. The activities of the animal, 

 however, differ widely from those of the plant. But the main 

 difference is in the nature of the food, and the manner of its 

 intus-susception. Plants can build up protoplasmic matter out 

 of such inorganic materials as contain the requisite elements. 



