8 HUMAN PHYSIOLOGY 



to that of the mother-organism. The mother-organism 

 transmits by heredity its characteristics to the daughter- 

 organism. 



The species of organisms existing at present have not 

 always existed since the beginning of life, but have, in the 

 course of time, been developed from simpler forms of life 

 (development of species, phylogeny). 



The forms which the growing organism assumes during 

 ontogeny are similar to the forms which the adults possessed 

 successively during the phylogeny. The ontogeny is a 

 short recapitulation of the phylogeny (biogenetic law of 

 Haeckel). 



The cause of morphological development lies in the vari- 

 ability of the structure and functions of the living being, i.e. 

 in the quantitative and qualitative variability in the trans- 

 formation of energy and matter. The cause of this variability 

 is not known. 



The principle according to which Darwin's theory of selection 

 explains the origin of species on the ground of variability is the 

 " natural selection in the struggle for existence. " The individuals 

 of a generation of a certain species differ slightly because of their 

 variability in structure and functions. Now, the struggle for 

 existence which the individual carries on with hostile beings of 

 the same or other species is endured best by those individuals 

 which have the most advantageous characteristics. These indi- 

 viduals are therefore sooner selected for further existence and for 

 reproducing offspring which inherit their characteristics. Such a 

 selection, carried on through many generations, at last produces 

 organisms which possess that characteristic developed to such an 

 extent that they really differ from their ancestral organisms. 



The continuous selection of advantageous variations leads to 

 the "development of beings highly fitted for their environment. 

 In this manner originates the adaptation which we see in so many 

 organs and organisms. 



While the Darwinian theory explains the origin of species on 

 the basis of the law of variability, it sheds no light on the cause of 

 this variability; in other words, it does not explain the very con- 

 dition necessary for the origin of species and therefore cannot be 

 regarded as a complete explanation of phylogeny. 



The principle of the Darwinian theory is, no doubt, applicable 

 to the origin of many species. Whether it is by itself a sufficient 



