146 ERNST HAECKEL. 



Inheritance from a common ancestor causes the typical 

 agreement in form and structure which is met with in the 

 early stages of each class. Adaptability to the various sur- 

 rounding conditions of life causes the differences in form 

 and structure which the forms evolved therefrom exhibit in 

 the different species of each class. 



Inheritance as a physiological function appertains to the 

 phenomena of reproduction. Adaptability as a physiological 

 function appertains to the group of phenomena of nutrition 

 as is pointed out in detail in the nineteenth chapter of the 

 * General Morphology ' (p. 148—294). 



Phylogenesis is the mechanical cause of ontogenesis. In 

 this single proposition our principal monistic conception of 

 organic development is clearly pointed out, and on the verity 

 of this principle the truth of the Gastraea-theory pre-eminently 

 depends. The consequences of this will unfold themselves a 

 little further on. For or against this proposition must every 

 naturalist in the future decide who, not satisfied with merely 

 wondering at the remarkable phenomena occurring in Bio- 

 genesis, will aspire beyond this to understand their full signi- 

 ficance. By this proposition, at the same time, is the never- 

 to-be-filled-up gulf marked out which separates the older 

 teleogistic and dualistic from the newer mechanical and 

 monistic morphology. If the physiological functions of 

 inheritance and adaptability are pointed out as the only 

 causes of organic structure, so therewith, at the same time, 

 every species of teleology, or of dualistic and metaphysical 

 speculation, is withdrawn from the realm of biogenesis, and 

 the sharp antithesis between the leading principles is there- 

 with clearly defined. Either there exists a direct and causal 

 connection between ontogenesis and phylogenesis, or there 

 does not. Either ontogenesis is a concise abstract of phylo- 

 genesis or it is not. Between the acceptance of these two 

 there is no third ! Either epigenesis and descent, or pre- 

 formation and creation. 



In reference to this decided alternative. His deserves special 

 notice, because he has repeatedly and definitely pronounced 

 himself against our fundamental laws of biogenesis, and 



vidual repeats during the rapid and short course of its individual develo))- 

 nient the most important of those changes of form which its progenitors 

 liave passed through duriug the slow and long courses of their palseonto- 

 logical development, according to the laws of inheritance and adaptability. 

 This true fundamental law of organic development is the indispensable basis, 

 upon which rests the whole inner concord of the history of develop- 

 ment. I repeat it here, because, firstly, its acceptation is required for the 

 understanding of the following discussion ; and, secondly, because it is still 

 combated by many respected naturalists. 



