2 PROFESSOR A. M. MARSHALL. 



actual study of the processes of development ; the gradual building up 

 of the embryo, and then of the young animal, within the egg ; the 

 fashioning of its varions parts and organs ; the devices for supplying 

 it with food, and for ensuring that the respiratory and other interchanges 

 are duly performed at all stages : all these are matters of absorbing 

 interest. Add to these the extraordinary changes which may take 

 place after leaving the egg, the conversion, for instance, of the aquatic 

 gill-breathing tadpole — a true fish as regards all essential points of its 

 anatomy — into a four-legged frog, devoid of tail, and breathing by lungs ; 

 or the history of the metamorphosis by which the sea-urchin is gradually 

 built up within the body of its pelagic larva, or the butterfly derived 

 from its grub. Add to these again the far wider interest aroused by com- 

 paring the life histories of allied animals, or by tracing the mode of 

 development of a complicated organ, e.g. the eye or the brain, in the 

 in the various animal groups, from its simplest commencement, through 

 gradually increasing grades of efficiency, up to its most perfect form as 

 seen in the highest animals. Consider this, and it becomes easy to 

 understand the fascination which embryology exercise over those who 

 study it. 



But all this is of trifling moment compared with the great generali- 

 sation which tells us that the development of animals has a far higher 

 meaning ; that the several embryological stages and the order of their 

 occurrence are no mere accidents, but 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 us that as 

 individual animals arise, not spontaneously, but by direct descent from 

 pre-existing animals, so also is it with species, with families, and with 

 larger groups of animals, and so also has it been for all time ; that as 

 the animals of succeeding generations are related together, so also are 

 those of successive geologic periods ; that all animals, living or that 

 have lived, are united together by blood relationship of varying near- 

 ness or remoteness ; and that every animal now in existence has a 

 pedigree stretching back, not merely for ten or a hundred generations, 

 but through all geologic time since the dawn of life on this globe. 



The study of Development, in its turn, has revealed to us that each 

 animal bears the mark of its ancestry, and is compelled to discover its 

 parentage in its own development ; that the phases through which an 



