THE MODERN STUDY OF ZOOLOGY 23 



those extinct forms must be invaluable. Still it 

 must be borne in mind that it is only certain 

 animals, and of these animals only certain parts, that 

 are capable of being preserved as fossils, so that 

 palaeontology can only help us as regards certain 

 groups of the animal kingdom, and even concerning 

 these its evidence must necessarily be very im- 

 perfect and fragmentary. Indeed, had we only 

 these three methods to aid us, the attempt to re- 

 construct the pedigree of the animal kingdom could 

 only be successful to a very partial and limited 

 extent. Fortunately there is another method which 

 supplies us with evidence of the very kind we 

 most want, and which bids fair to far outweigh 

 in importance the other three methods. This is : 



4. Embryology, or the study of the actual de- 

 velopment of existing forms, the several changes 

 which they undergo during their gradual evolution 

 from the ovum. This, which is the second of the 

 two new influences referred to in a preceding page, 

 is, like palaeontology, a young science, but one that 

 has thriven mightily of late years. It was not till 

 towards the middle of the present century that it 

 began, in the hands of von Baer, to assume its 

 modern form, and it is only of recent years that it 

 has revealed its enormous powers as an instrument 

 for the solution of the hard problems of phylogeny. 

 Von Baer propounded the doctrine of animal types 

 quite independently of Cuvier ; and he went further 

 than his illustrious contemporary, for he showed 

 that not only as far as structure was concerned 

 might animals be arranged under certain definite 



