ANIMAL PEDIGREES 201 



number of generations, but of all animals and for 

 all time : a formidable, but an entrancing problem; 

 and whatever misgivings I may have as to my 

 power of presenting it aright, no apology is needed 

 for asking your attention to a consideration of the 

 means at our disposal for attacking it, of the evi- 

 dence on which we rely in our attempts to recon- 

 struct the past histories, to determine the pedigrees 

 of animals. 



On the present occasion, it is not with the 

 whole evidence, but with one special side of it, that 

 we shall be concerned, that namely which is 

 derived from a study of the development of existing 

 animals. Every one knows that animals in the 

 earlier stages of their existence differ greatly in 

 form, in structure, and in habits from the adult 

 condition ; a lung-breathing frog for example 

 commences its life as a gill-breathing tadpole ; and 

 a butterfly passes its infancy and youth as a 

 caterpillar. It is clear that these developmental 

 stages, and the order of their occurrence, can be no 

 mere accidents ; for all the individuals of any 

 particular species of frog, or of butterfly, pass 

 through the same series of changes. It is not 

 however until recent years that naturalists have 

 realised that each animal is constrained to develop 

 along definitely determined lines ; and that the suc- 

 cessive stages in its life-history are forced on an 

 animal in accordance with a law, the determination 

 of which ranks as one of the greatest achievements 

 of biological science. 



The doctrine of Descent, or of Evolution, teaches 



