290 BIOLOGY 



for example, the embryo pursues the most direct course pos- 

 sible for building itself from the simple egg to the adult, and 

 the chick, when it hatches from the egg, is practically adult 

 in form, although not in size. In such cases we call the history 

 a direct development, in contrast to an indirect development 

 or a metamorphosis. 



Embryology a Repetition of Past History. It will be seen 

 from the development of the frog that at one period it resembles 

 a fish in a number of points. It lives in the water, has a flat 

 swimming tail, possesses branchial slits, and carries on respira- 

 tion by means of gills. The study of geology has shown that 

 in the history of the world fishes preceded frogs, and it is thus 

 seen that in its embryology a frog shows a tendency to repeat 

 the past history of animals. Such a repetition is found, not 

 only in the frog but in many other animals, for it is a funda- 

 mental biological law that embryology repeats past history. 

 In technical terms this is expressed by the statement that on- 

 togeny is a repetition of phylogeny, ontogeny (Gr. on = being + 

 -geneid) being the individual's embryological history, and phy- 

 logeny (Gr. phylon = tribe + -geneia = producing) the history of 

 the race, during the geological ages. This parallel has been one 

 of the strong arguments which have convinced scientists that our 

 present forms have been derived by ordinary methods of de- 

 scent, through the process of reproduction, from the earlier in- 

 habitants of the world; or, in other words, that the history of 

 the organic world has been one of evolution and not one of spe- 

 cial- creation of each species independently, as was formerly be- 

 lieved. While a few years ago this law of repetition was thought 

 to be more strictly adhered to than careful study has proved to 

 be the case, the general fact that embryology tends to repeat 

 past history remains as one of the interesting and significant 

 laws of nature. It is sometimes called the biogenetic law. 



Oviparous and Viviparous Animals. Many animals (for ex- 

 ample, the frog) extrude their eggs into the water as soon as they 

 are mature and take no further care of them. In some cases, 



