212 THE PHILOSOPHY OF BIOLOGY 



of biology shows us that all animals pass through a 

 series of stages in their individual development, or 

 ontogeny. The earlier stages represent a simple type 

 of structure, usually a hollow ball of cells, but as 

 development proceeds, the structure of the embryo 

 becomes more and more complex. The process of 

 development is continuous in many animals, but in 

 others (perhaps in most) larval stages appear, that is, 

 development is interrupted, and the animal may lead 

 for a time an independent existence similar to that of 

 the fully developed form. Often these larval stages 

 suggest types of structure lower than that of the 

 fully developed animal into which they transform. 

 Even if larval stages may not appear in the ontogeny, 

 it is very often the case that the developing embryo 

 exhibits traces, or at least reminiscences, of the types 

 of morphology characteristic of the animals which 

 are lower or less complex than itself ; thus the piscine 

 gills appear during the development of the tailed 

 Amphibian, and even in that of the Mammal, and then 

 vanish, or are converted into organs of another kind. 

 The individual thus passes through a series of develop- 

 mental stages of increasing complexity : it repeats, 

 in its ontogeny, the palaeontological sequence in a 

 distorted and abbreviated form. 



It is true that the evidence afforded by palaeontology 

 is very meagre. The preservation of the remains of 

 organisms in the stratified rocks is a very haphazard 

 process, and it depends for its success on a series of 

 conditions that are not always present. As the 

 surface of the earth becomes better known, our know- 

 ledge of the life of the past will become fuller, but there 

 can be little doubt that whole series of organisms 

 must have existed in the past, and that no recognisable 

 traces of these are known to us. There is also no 



