46 PHYSIOLOGY CHAP. 



Darwin's book On the Origin of Species T)ij Cleans of Natural 

 Selection caused a radical change in the ideas of the naturalists, 

 and led to the almost unconditional triumph of the law of evolu- 

 tion, to the detriment of the law of stability. The evolutionists 

 fell into excesses, even denying the existence of biological species. 

 We have recently entered upon a period of acute criticism of old 

 and new theories of the Origin of Species, and at present the con- 

 viction is gaining ground that none of these theories has an 

 .absolute demonstrative value, all having rather the significance of 

 hypotheses that are of great use to the biologist in orientating 

 himself in his positive researches. 



The idea of evolution has till now been the only conception 

 imagined by the naturalists to account for the evident affinity 

 exhibited among themselves by the different plants and animals 

 which are grouped into species, genera, families, orders, classes. 

 In all these groups a certain conformity of morphological type is 

 apparent. 



According to the Evolutionary Theory, this unity of type is 

 the expression of a unity of origin (monophyletic origin), from 

 which the various families, genera, and species, animal and veget- 

 able, have been derived by successive differentiations. Compara- 

 tive anatomy, embryology, palaeontology, botanical and zoological 

 geography, offer numerous facts that accord perfectly with the 

 theory of evolution. With the progress of biological science, 

 however, other data have gradually emerged that are difficult to 

 reconcile with the concept of simple, continuous, monophyletic 

 evolution. 



Many of the resemblances, analogies, and honiologies admitted 

 by comparative anatomists up to a few years ago are no longer 

 valid in face of a more profound and exact knowledge of the true 

 structure and function of certain organs that were previously 

 imperfectly known. For embryologists, the value of the so-called 

 " great biogenic law " that was held by certain naturalists to be 

 one of the fundamental proofs of evolution, has depreciated owing 

 to the many exceptions which it presents. Further, the analogy 

 between the development of the individual (ontogenesis) and the 

 development of the species (phylogenesis) is essentially different, 

 since the cell-ovum from which the individuals of the evolved 

 species originate differs entirely from the ovum of the Protista, and 

 must in itself (by a still incomprehensible mystery) contain the 

 whole of the determinants of the complex final development, 

 determinants that are obviously wanting in the ovum of Protista, 

 or are contained there in a far less degree. 



Nor, again, have recent palaeontological data provided all the 

 arguments in favour of the theory of evolution that were claimed 

 a few years ago. Nowadays we can no longer invoke insufficiency 

 of material to explain the great lacunae found in the development 



