THE GENERAL CONDITIONS OF LIFE 317 



highest differentiation of the plants. From the Protozoa, on the 

 other hand, arose the Gastrmada, very simple animals consisting of 

 only two layers of cells (entoderm and ectoderm). Probably no 

 representative of this group is now living, but their presence in 

 the genealogical series must necessarily be inferred from the very 

 general appearance of the gastrula-stage in the development of 

 all animals. From the Gasirceada developed on the one side 

 polyps, jelly-fishes, etc. {GKlenterata), and upon the other side 

 worms ( Vermes). The latter gave origin to four groups comprising 

 respectively star-fishes, sea-urchins, etc. (JEchinodermata), crabs, 

 insects, etc. (Arthropoda), tunicates (Tunicafa), and clams, snails, 

 etc. {Molliisca). Of these the tunicates became the j^rogenitors of 

 the back-boned animals ( Vcrtchratd), the most widely differentiated 

 representatives of the animal kingdom. The present living organ- 

 isms form merely the last shoots of all the branches of this great 

 genealogical tree. 



A glance over the racial development of organisms from their 

 first appearance down to the present time shows that living sub- 

 stance has undergone in the course of the earth's development a 

 remarkable change in form and organisation ; in these respects, 

 existing organisms are widely differentiated in very different 

 directions. 



Darwin's theory of selection has afforded a natural explanation 

 of this phenomenon. This theory proceeds from the fact that all 

 individuals of the same species, even descendants from the 

 same pair of parents, differ from one another more or less 

 markedly. This phenomenon is known as individucd variahility, 

 and is the result partly of sexual intermixture (Weismann's 

 amphimixis) and partly of the action of various external influences 

 upon the germ-plasm of the individual embryos, whether within or 

 without the maternal organism. Of these more or less different 

 individuals of the same generation Darwin shows that in the 

 struggle for existence only those continue to live that are best 

 fitted to the external conditions of life, while those that are less 

 fitted perish as a result of the competition with the former. Thus, 

 only those that are best adapted to the existing external conditions 

 can reproduce and transmit their characteristics to their descend- 

 ants. In this survival, in this selection of the fitter individuals, lies 

 the naturcd selection of Darwin; and it is evident that with the 

 continuance of the process organisms must become adapted to 

 existing vital conditions very perfectly. Hence the form, the 

 organisation, and, in general, all the characteristics of living 

 substance are in the closest correlation with the external con- 

 ditions upon the earth's surface ; if these change, the characteristics 

 of organisms must correspondingly change. 



But it is a question whether, in the course of time, natural 

 selection is the sole factor that causes organisms to change. 



