3o8 NATURAL SCIENCE. May, 



genealogical tree must correspond essentially with the ontogenetic 

 stages of a determined course of development. If the biogenetic 

 law were correct, embryology would thus be in a position to re- 

 construct, at least approximately, the primitive forerunners of each 

 group of plants and animals ; and these types should, if they were 

 capable of preservation, also lie buried in the rocks. 



If we consult palaeontology, it shows that these surmises are by 

 no means confirmed. There are, indeed, a great number of fossil 

 genera which retain throughout life the embryonic, or, rather, the 

 youthful characters of their existing allies, but it is only among the 

 mammals, and to some extent among the reptiles, that I could name a 

 complete series of forms following one another in time and belonging 

 to the same line of development. The Eocene, Oligocene, and, in 

 part also, even the Miocene Mammalia, stand to their now existing 

 allies, for the most part, in the relation of youthful forms, while they, 

 almost without exception, exhibit at least some characters which are 

 quickly passed through by their geologically younger successors in the 

 embryonic or youthful stage. On the other hand, they are, as a rule, 

 destitute of the most striking peculiarities, such as antlers, bony 

 processes, fusion of certain bones, reduction of the teeth or of 

 separate parts of the skeleton ; and it is not till we study more closely 

 a series of related genera of different geological ages that we see how 

 the differentiations and peculiarities of the existing representatives of 

 any particular group have been gradually formed in course of time. 

 But thus it is also possible to discover, in most of the mammalian 

 orders, a number of primitive characters, which, while they frequently 

 occur united in the oldest representatives of the group in question, 

 also usually correspond to an embryonic stage of one of its living 

 members. 



The ontogeny of organisms now living would, for the rest, afford 

 but an exceedingly unsafe basis for the reconstruction of ancient 

 faunas and floras, since experience teaches that the biogenetic law is 

 frequently veiled or completely obscured owing to various causes. 

 Not seldom does it happen that, of two nearly allied living forms, the 

 one passes through a series of continuous, successive stages, while 

 development in the other takes place more by jumps. In the latter 

 case the embryo is driven by peculiar influences to an acceleration of 

 its development ; it completely jumps over certain stages, and thus 

 renders unintelligible the historical (palingenetic) record preserved in 

 the ontogeny of each individual. This falsification of development — or 

 Ccenogenesis, as Haeckel calls it — chiefly occurs when the adult 

 individual manifests a high degree of differentiation, and when the 

 embryo has to pass through considerable changes to reach its final 

 form. How unsafe and deceptive palaeontological results would be if 

 attained by embryological paths may be gathered from some random 

 instances. What wonderful ancestors would be constructed for the 

 crinoids by a zoologist who only knew the life-history of Antedon ! 



