12 THE STUDY OF ANIMAL LIFE CHAP. 



will remain indeed to connect the living twigs with 

 those whose leaves fell off ages ago, to understand the 

 continual renewal of the foliage by the emergence of 

 new leaves, and finally to understand how the entire tree 

 of life has grown to be what it is. 



There is of course no doubt as to the fact that some 

 forms of life are more complex than others. It requires 

 no faith to allow that the firstlings or Protozoa are 

 simpler than all the rest ; that sponges, which are more 

 or less loose colonies of unit masses imperfectly com- 

 pacted together, are in that sense simpler than jellyfish, 

 and so on. The animals most like ourselves are more 

 intricate and more perfectly controlled organisms than 

 those which are obviously more remote, and associated 

 with this perfecting of structure there is an increasing 

 fulness and freedom of life. The standards which we use 

 in calling animals " high " or " low " are two : the first 

 is differentiation, which means complexity of structure, 

 the second is integration, which means unification of 

 structure. 



We may arrange all the classes in series from low to 

 high, from simple to complex, but this will express only 

 our most generalised conceptions. For within each class 

 there is great variety, each has its own masterpieces. 

 Thus the simplest animals are often cased in shells of 

 flint or lime whose crystalline architecture has great 

 complexity. The simplest sponge is little more than a 

 double-walled sack riddled by pores through which the 

 water is lashed, but the Venus' Flower-Basket (Euplec- 

 tella), one of the flinty sponges, has a complex system of 

 water canals and a skeleton of flinty threads built up 

 into a framework of marvellous intricacy and grace. The 

 lowest insect is not much more intricate, centralised, or 

 controlled than many a worm of the sea-shore, but the 

 ant or the bee is a very complex self-controlled organism. 

 More exact, therefore, than any linear series, is the image 

 of a tree with branches springing from different levels, 

 each branch again bearing twigs some of which rise 

 higher than the base of the branch above. A perfect 

 scheme of this sort might not only express the facts of 



