148 ERNST HAECKEL. 



anatomy and the referring the ontogenetic processes to their 

 mechanical phylogenetic causes — their true " causae effi- 

 cientes." If His had only known a little of the facts of 

 comparative anatomy and of the ontogenesis of Invertebrata 

 he would scarcely have published his essays. 



To perceive quite clearly the complete antithesis between 

 this pretended exact "physiological "conception of ontogenesis 

 and our antecedent explanation of the same by phylogenesis, it 

 is only necessary to compare these abortive inquiries of His's 

 with the masterly outline of the history of the development of 

 the Crustacea which Fritz Miiller has given in his eminently 

 suggestive work, ' Fiir Darwin ' (Leipzig, 1864), Here the 

 immediate dependence of ontogenesis on phylogenesis is 

 proved from the multifarious range of forms of one whole 

 class of animals, and the former is actually explained by 

 means of the latter. 



Here we find both the formative forces — inheritance and 

 adaptability — referred to as the true " physiological" causes 

 of ontogenesis and the laws regulating their activity recog- 

 nised. Two of the most important propositions which Fritz 

 Miiller herein lays down, and which have a special bearing on 

 our present subject, are as follows : — The historical witness 

 (of ancestral development), preserved in the developmental 

 history (of the individual), will gradually become obliterated. 

 Such development always follows a direct path, from the ovum 

 to the perfect animal, and it will frequently, in the struggle 

 for life which the free living larval form must undergo, put 

 on a deceptive appearance. 



The past history of the species (phylogenesis) would be so 

 much the more fully preserved in its developmental history 

 (ontogenesis) in proportion to the duration of its younger 

 stages, which also themselves pass througli similar phases, 

 and so much the more truly the less the life-habits of the 

 young deviate from those of the adult forms, and the less 

 the peculiarities of the individual younger stages present 

 themselves as retrogression from a later to an earlier division 

 of life, or as independent entities (' Fiir Darwin,' pp. 77-81). 

 Whilst Fritz Miiller founded these laws on the ontogenesis of 

 various Crustacsea, and from the common nauplius young 

 form of the most different kinds thereof, he concludes that a 

 common nauplius form was the ancester of the whole class, 

 and he explains, at the same time, a number of remarkable 

 phenomena, which, without this application of the theory of 

 descent, would remain perfectly unexplicable, not to say 

 incomprehensible. The causal bearing of phylogenesis on 

 ontogenesis may be at once perceived therefrom. 



