178 GENERAL PHYSIOLOGY 



are termed their development. Two great series of form-changes 

 are recognised in living substance — phylogenetic or racial develop- 

 vient, which comprises the form-changes of living substance in 

 their totality during the earth's development ; and ontogenetic or 

 germinal development, which comprises the form-changes that a 

 single individual goes through during his life. Haeckel ('66), 

 who has done pioneer work of fundamental importance for the 

 modern theory of evolution, has shown that the two series stand 

 in intimate connection with one another; in general, germinal 

 development is an abbreviated recapitulation of racial develop- 

 ment. 



A. PHYLOGENETIC DEVELOPMENT 



The forms of living substance that inhabit the earth's surface 

 have not always been the same. Modern palaeontology, the 

 science of fossil organisms, has revealed an overwhelming number 

 of forms which differ from those now living the more the older the 

 strata from which they are derived. Critical research during the 

 last decade has relegated to the realm of fable a large number of 

 remarkable beings with which the earlier geology peopled the earth, 

 and has shown them to be fanciful pictures which stand upon the 

 same plane as the rare animal forms contrived by the curious creative 

 fancies of the Indians, the Assyrians, and the Incas ; nevertheless, 

 the discovery of well-authenticated fossil forms during recent de- 

 cades has proved conclusively how utterly different from its present 

 state was the organic world upon the earth's surface during the 

 earlier periods of the earth's development. An overwhelming- 

 number of organisms have become known which inhabited the 

 water and the land before man. The theory of descent has intro- 

 duced a causal connection into this wealth of forms by showing that 

 fossil organisms are not to be regarded as unique curiosities, h(sus 

 naturce, and the unsuccessful experiments of a Creator, as the 

 previous century believed them to be. Rather are they the dead 

 twigs and branches of a mighty, wide-spread trunk, of which the 

 youngest and last shoots are the present living organisms ; the 

 oldest branches have sprung from a common root, the Protista, 

 whose direct descendants, little changed, now appear in the in- 

 teresting groups of unicellular beings, Bhizopoda, Bacteria, Infu$mna, 

 and Algm. Modern morphology has succeeded by critical research 

 in drawing in gross outline a picture of the genealogical tree of 

 organisms, and the conception of natural relationship, which was 

 presaged by the use of the word by the earlier systematic mor- 

 phology in a figurative sense, has obtained through phylogenetic 

 research a very real significance. The present organic world 

 is the product of an historic development stretching back o-\er 

 an enormously long space of time, in which some forms, such as the 



