22 THE ORGANIC GROWTH OF THE LIVING WORLD SEC. 



The causes I formerly called " internal," therefore, have 

 nothing to do with the internal causes of Nageli, a fact which 

 is proved without a doubt by his latest exposition, and for the 

 future I shall avoid the expression. 



FUNDAMENTAL CAUSES OF THE MANIFOLD VAKIETY OF 

 OKGANIC FORMS 



According to my conception, the physical and chemical 

 changes which organisms experience during life through the 

 action of the environment, through light or want of light, air, 

 warmth, cold, water, moisture, food, etc., and which they 

 transmit by heredity, are the primary elements in the pro- 

 duction of the manifold variety of the organic world, and in 

 the origin of species. From the materials thus supplied the 

 struggle for existence makes its selection. These changes, 

 however, express themselves simply as growth. 



As individuals grow so the whole world of organic forms 

 has grown up from simple beginnings. 



To separate the continuous growing whole into parts, 

 into species, required, and still requires, special means after- 

 wards to be discussed. Let us first of all in what follows 

 exclude the necessity of this separation. 



Warmth, air, light, moisture, food, condition the growth of 

 the individual being appear before our eyes as the mightiest 

 impulses which determine the manifold variety of the forms 

 of living beings. They condition growth through physical 

 and chemical change of the living organic mass, the plasma, 

 through the formation of new and more complex compounds. 



Since the external conditions have not always remained 

 the same, but have changed in the course of ages upon our 

 earth, and since they are even now locally different, so that 

 one and the same organism at different parts of the earth, in 

 different dwelling-places even of the same region of the earth, 



