358 THE HISTORY OF BIOLOGY 



for his expressly ascribing the abundance of species in the insect class to 

 hybridization. 



Law of unity 

 All the speculations just referred to, Meckel includes in the sphere of the 

 law of multiplicity. As already mentioned, under the law of reduction come 

 the proofs of the unity of the life-type. Here Meckel displays the whole of 

 his extensive knowledge of anatomy, both good and bad; here he presents 

 his most brilliant ideas and also indulges in the most ridiculous absurdities. 

 The latter necessarily result from the false preconception of one single life- 

 type and evolutional series, the same fatal constructions of thought, in fact, 

 which led Lamarck and GeofFroy to such wild delusions. And it may be said 

 that herein Meckel outrivals them to the same extent as his anatomical 

 knowledge is more extensive than theirs. It is hardly worth while going 

 further with him along these erratic courses; as when he compares the shell 

 of the tortoise and of insects, or the papillas on the tongue of cats and of 

 snails, or when he likens the double malformations occurring in man, now 

 generally called Siamese twins, to a colony of polypi. It would be better to 

 ponder over the numerous ideas of great value for the future to which he gives 

 expression in this connexion. Among these ideas that have been generally 

 adopted in modern times may be mentioned his view that the lungs of the 

 land vertebrates are derived from the air-bladders of fishes, his comparison 

 between the male and the female sexual organs and his derivation of their 

 several parts from an indifferent embryonic stage, his comparison of the 

 segmentation of worms and articulates with metamerism in the vertebrates, 

 and, above all, his foreshadowing of Haeckel's biogenetical organic law, 

 when he declares that each higher animal during its embryonic development 

 passes through the same forms as those that are lower in the evolutional 

 series possess when fully formed. This theory of "the development of the 

 special organism in accordance with the same laws as the entire animal 

 series" he supports by a number of very ill-founded arguments — for in- 

 stance, he makes the human liver undergo a crayfish and a mollusc stage — 

 but also on reasoning which has been accepted by modern supporters of the 

 theory. This doctrine, which had already been sweepingly rejected by Rudol- 

 phi, has certainly been very widely debated in our own day, but at all events 

 it has had a highly stimulating effect upon research; its subsequent fate will 

 be discussed later on. The theoretical conception that Meckel thus formed 

 he applies in detail in the special part of his work wherein, with a many- 

 sided and at the same time thorough knowledge of his subject, which but 

 few had so far emulated, he describes the structure of the organic systems 

 throughout the animal kingdom. With extreme thoroughness he discusses 

 and compares the bone-structure in the vertebrates, describing especially 

 the bones of fishes in great detail and making new discoveries in this latter 



