362 



SCIENCE. 



[N. S. Vol. III. No. 62. 



ing of the forms, and is fully comparable to ar- 

 tifical selection. 



Ortmann ends his discussion with the follow- 

 ing resume: From these considerations it be- 

 comes evident that four factors contribute to 

 the formation of different species: 1. Adapta- 

 tion to external conditions produces variations. 2. 

 The inheritance of these adaptations fixes the varia- 

 tions and shapes groups of forms morphologically 

 related. 3. Natural selection modifies the groups 

 and produces mutation in a certain direction. 4. 

 The isolation of groups produces differentiation in 

 the direction of mutation and therefore formation of 

 new species. All these If factors must cooperate; none 

 can he absent, and none is possible without the others. 



Amphimixis operates in a conservative man- 

 ner on the average characters, thus leveling the 

 variations, which are capable of preservation, 

 and which are not, therefore, injurious. 



The principle of separation has an important 

 bearing on zoogeography, since it follows that 

 species must originate in isolated localities; they 

 are bound to centres of origin. We want to know 

 the place of origin of a given species which at 

 present lives in a certain locality. Did it origi- 

 nate or immigrate there. From this it follows, 

 that in the fauna of each single locality we have 

 to distinguish: First species which originated 

 there, autochtons, and second species which 

 immigrated from other localities. Immigrants. 

 A third group are the relicts, which formerly 

 had an extensive distribution, but are now re- 

 stricted to a few points. The decision of the 

 nature of a certain animal form in a certain 

 region, whether autochton, immigrant or relict, 

 can only be given by systematics, and here the 

 slightest detail must be considered. At this 

 point zoogeography is not only most intimately 

 connected with systematics, but entirely de- 

 pendent on it. 



The following pages discuss the law of the 

 continuity of the areas of distribution, and the 

 increase and prevention of distribution. 



According to the principle of migration, the 

 single animal forms can only extend to such 

 regions as are iu connection with the original 

 center: This is the law of the continuity of the 

 areas of distribution. It is a well known fact 

 that the range of every species extends over 

 a number of localities, which are separated 



from each other by smaller or larger gaps ; 

 these gaps, however, must not be so large that 

 they could not be surpassed by the species in 

 question; thus a continuous communication of 

 the inhabitants of the single localities in which 

 the species is found is possible. Only when 

 this is the case can we speak of a continuity of 

 life conditions. As soon, however, as this con- 

 tinuity is interrupted in such a way that it pre- 

 vents the communication of the species, a barrier 

 is formed which prevents its further extension. 

 Continuity of life-conditions increases the dis- 

 tribution of animals; their interruption pre- 

 vents it. 



There are especially two factors which are of 

 importance in this connection, for the first time 

 clearly defined by Pfeffer, the climatological and 

 the topographical; a third one has been added by 

 Ortmann — the biological. 



The effect of climatic conditions on the distribu- 

 tion of animals. 



A uniform distribution of animals in their life 

 districts presupposes uniformity of the climatic 

 conditions, since all animals are highly dependent 

 on temperature. The importance of the condi- 

 tions of temperature was first pointed out by 

 Dana ; he considered the minimal absolute alti- 

 tude which the animals need as the most impor- 

 tant point, and constructed his Isocrymes, lines of 

 equal lowest temperature. That the principle 

 is not correct is generally admitted at present, 

 and it has been replaced by another one. It is 

 not so much the absolute altitude of the tem- 

 perature which influences animal life, but it is 

 the Amplitude, the amount of oscillation, since 

 the temperature in the same locality oscillates 

 according to the seasons. Mobius therefore has 

 distinguished stenothermous and eurythermous 

 animals. Stenothermous animals are unable to 

 stand considerable oscillations ; they are bound 

 to a more uniform temperature ; eurythermous 

 animals are not affected by considerable changes. 

 The fundamental difference between the marine 

 and continenttil conditions of temperature is 

 thereupon discussed. On the continents we 

 have high amplitudes, the surface temperatures 

 of the ocean being more uniform. Since the 

 terrestrial animals are adjusted to high ampli- 

 tudes — ^being eurythermous — their distribution 

 is not so much influenced by the climatic differ- 



