Chap. XX.] THE AMCEBA. 357 



And not only (1) do the parts of an organism get more different 

 and yet at the same time more intimately connected, but the 

 organism (2) tends to become more different from the environ- 

 ment, and yet, at the same time, requires to be, so to speak, more 

 closely in touch with the conditions of the environment ; and yet 

 further, (3) the organism not only becomes more different from 

 other organisms, but at the same time becomes more dependent 

 upon other organisms, so that the whole organic world becomes 

 more and more a vast system of interdependent units. 



These considerations help us to see what is the meaning of 

 the terms higher and lower as applied to animals. The higher 

 animals are those in which this joint process of differentiation 

 and integration has been carried furthest. But it is well to 

 avoid instituting comparisons of this nature between animals of 

 widely different groups. Such a question as, Which is the 

 higher animal a bee or a bear ? is not very profitable. It is 

 even well to limit comparison, so far as possible, to analogous 

 cases, and to speak of higher and lower in respect to the per- 

 formance of special functions. At the same time we may per- 

 haps say generally that the more complex the conditions of life 

 to which an organism is suited, and the more perfectly that 

 organism is suited to these conditions of its life, other things 

 equal, the more advanced it is. 



A final question arises out of the fact that if we prepare an 

 animal or vegetable infusion (e.g. hay or cod's head), and leave it 

 to stand for a short time, we shall find it swarming with organ- 

 isms paramoecia, amoebae, monads (Fig. 103, C. D.) and countless 

 bacteria. Whence comes all this teeming life ? Two answers 

 are or have been given to this question. The first (abiogenesis) 

 is that the living forms take their origin by a regrouping of the 

 molecules of the organic but lifeless matter in the solution, with- 

 out the aid of any parental organisms. The second (biogenesis) 

 is that the living forms are due to the presence in the solution of 

 organic living germs, the products of parental organisms. Into 

 the arguments brought forward by the advocates of these two 

 answers we cannot enter here. The balance of scientific evi- 



