482 PHILOSOPHIC ZOOLOGY 



relatives, we find in the Australian Duck- Bill (^Ornithorhynchus) 

 that the adult animal possesses four horny plates in place of 

 teeth. But these are preceded by small molars (fig. 1336) which 

 last for a short time only. The conclusion may be drawn that 

 the Duck- Bill is descended from ancestors which possessed teeth 

 when adult. And, in connection with this, it is interesting and 

 significant that the transitory teeth of this creature are singularly 

 like those possessed by an extremely ancient Mesozoic mammal, 

 which has been extinct for an enormous length of time. 



Argument from Development. — As already explained in 

 the section on Development and Life- History (vol. iii), an 

 animal of complex structure results from a process of gradual 

 up-building, in which the ovum or egg -cell is the first and 

 simplest stage. Speaking very broadly, the course of this 

 development is taken to be a recapitulation of the history of 

 the group to which the particular animal belongs. The life- 

 history of a particular form may, for example, include stages 

 adapted to different modes of life, and in some cases these 

 apparendy correspond to ancestral stages similarly adapted. We 

 see this in the Frog, which is hatched out as an aquatic tadpole, 

 breathing by gills and fitted in various other ways for life in 

 water. From this the conclusion is drawn that the remote 

 ancestors of Frogs were aquatic creatures, related to the stock 

 from which recent fishes have descended. The argument may 

 be extended to Reptiles, Birds, and Mammals, for though these 

 do not begin life as aquatic tadpoles, all of them possess gill- 

 slits during certain stages in their development. But these 

 slits have nothing to do with breathing, apparently serving no 

 useful purpose, and ultimately close up. One result of their 

 presence in the embryo is that some of the blood-vessels develop 

 in a somewhat roundabout manner (see vol. i, p. 244). These 

 vessels begin in conformity to what may be called the "fish- 

 plan ", abandoning this later on for the arrangement char- 

 acterizing the air-breathing adult. Such a peculiar method of 

 development is quite unintelligible unless it is explained by 

 reference to ancestry. 



The F"eather-Star {Coiiiatuld) furnishes a striking example 

 of recapitulation in its life-history. It is for some time fixed to 

 some firm object by means of a stalk, which is later on aban- 

 doned. This may very reasonably be taken to mean that 



