ISO GEOLOGICAL BIOLOGY. 



lows will, in the long-run, survive them in the race, and they 

 will increase and prevail while the others will drop out and be 

 lost. 



Definition of Ontogeny and Phylogeny. In the analysis of 

 Huxley's definition of evolution (or development) the two- 

 fold division of the history is adopted, which is expressed in 

 part by the terms Ontogenesis and Phylogenesis, introduced 

 by Haeckel. Haeckel briefly defined these terms, as follows: 

 Ontogeny, or Ontogenesis: The history of the development 

 of the individual (including Embryology and Metamorphol- 

 gy) ' Phylogeny, or Phylogenesis : The paleontological history 

 of the development of the ancestors of a living form. It is 

 proposed to restrict the term development to the meaning 

 expressed by Ontogenesis, and to restrict the use of evolu- 

 tion to Phylogenesis. In Ontogeny we find the individual 

 organism beginning with a great majority of its lines of 

 development or steps of metamorphism already determined 

 for it. Take, as an example, the crayfish, which Huxley 

 has so interestingly dissected and described,* and of it we 

 can say at the first stage of the embryo that in case it lives 

 at all, whatever the conditions of environment may be, it 

 will develop all the characters of the branch, class, order, 

 family, genus, and species to which it belongs. Its name, 

 Astacus fltiviatilis, applies to it in all stages of its develop- 

 ment from the embryo up (Fig. 50). 



The Main Features of Development Predetermined before they 

 Begin. We can predict before any trace of the characters 

 appear (with as great a degree of certainty as we can predict 

 the result of combining a given acid with a grain of chemical 

 salt) what the path of development will be which the embryo 

 will take if it continues to grow. It will surely develop a 

 jointed body, with the articulated limbs and chitinous crust of 

 the Arthropoda. It will surely develop a breathing apparatus 

 of gills situated on the maxilliped and legs of the Crustacea. 

 The appendages of the cephalothorax will certainly be an- 

 tennae, and the specialized biting mouth parts of the sub-class 

 Neocaridae, not the simple legs of the more ancient sub-class 



*"The Crayfish, an Introduction to the Study of Zoology" (Appletons, 

 1880). 



