ORTHOGENESIS 429 



amount of nourishment received by the parent. If 

 starved and highly nourished seeds a^e planted in the 

 same environment they respectively produce small and 

 large plants. It is important to note, however, that such 

 effects pass away in a few generations. The germ cells 

 have not been affected, and the modification is not in- 

 herited. The same facts have been shown to be true in 

 the case of certain organisms subjected to different degrees 

 of temperature, moisture, light, drugs, and several other 

 modifying agents. The effects of the environment in 

 such cases are not permanent, and hence are not heritable. 



On the other hand it is beyond question that muta- 

 tions do appear from time to time. It is also well estab- 

 lished that this is due to the rearrangement of the heredi- 

 tary chromatin in the germ cell. In some cases there is 

 evidence that such changes are closely related to tempera- 

 ture — an environmental factor which thus appears to 

 modify heredity. Since the germ cells are protoplasm, 

 and protoplasm is capable of change, it may indeed be 

 true that environmental influences have an effect. They 

 are believed to have by the later Lamarckian school, but 

 the question is as yet a hotly debated and unsettled one. 

 Even if it were settled in the affirmative, however, it is 

 probable that it would do no more than shed light upon 

 the origin of heritable variations, and would in no wise 

 unseat the theory of Natural Selection. 



Orthogenesis or Determinate Evolution. — The 

 evolution of the organic world may be compared to the 

 growth of a tree, the tips of the branches corresponding 

 to the modern species of animals and plants. And just 

 as the tree is caused to grow by forces within it, so, a 

 considerable number of scientists are convinced, living 

 things are constrained by their structure or by internal 

 or external forces or by these in combination to evolve 

 along fairly definitely fixed paths. Such in general is the 

 meaning of Orthogenesis. 



According to the most extreme view, protoplasm is en- 



