MODERN BIOLOGY 615 



his controversy with Lotsy that the species are abstract and arbitrary 

 ideas and he has quoted the statements of other observers in support of 

 this view. 



Mutations and hereditary factors 

 That the systematic species in the old sense of the term, comprising indi- 

 viduals having a certain mutual resemblance, must possess immense, even 

 inevitable, significance from a purely practical point of view, will at once 

 be realized; moreover, it has both morphological and physiological impor- 

 tance, in that resemblance of various kinds, both outward and inward, char- 

 acterize the same type of organism, from the simplest characters referred to 

 in the text-books to the precipitin reaction. But the species-characters — 

 mutual resemblance — say, as is established, nothing about mutual affinity, 

 and heredity research is the one branch of biology that cannot utilize the 

 idea of species, except under experimental control. The old speculations upon 

 species-phylogenesis, however, have not been given up by the representa- 

 tives of modern heredity-research; on the contrary, there is current among 

 them a veritable maze of ideas on the problem of origin. The problem has 

 been further complicated by a divergence of views regarding the relation 

 between mutations and Mendelian cleavage; while Heribert-Nilsson, for in- 

 stance, has found that mutations, where they exist, only cause loss of heredi- 

 tary factors, and on that account holds that the process of evolution can be 

 conceived merely as a series of reductions in the original material of genes, 

 the specialists in Drosophila have proved that there have existed mutations 

 which have given rise to positively new rudiments. The conception of the 

 development of life on the earth must of course be influenced by this prob- 

 lem. Heribert-Nilsson has resolutely followed up the consequences of his 

 theory that only detrimental mutations exist and has declared that a theory 

 of evolution is on the whole unthinkable, while other geneticists have 

 deemed it unnecessary to take refuge in this desperate expedient. Baur in 

 particular has endeavoured to create a theory of evolution by combining 

 the results of research work on mutations and hybridization with Darwin's 

 theory of selection; a similar view has been held by Morgan and his school. 

 Other students of heredity, again, have sought to establish a mutational 

 Lamarckism by assuming mutations induced by the influence of external 

 conditions upon the germinal plasm. Earlier attempts to prove the possi- 

 bility of transmitting to the offspring characters that have been experimen- 

 tally imparted to the parents either have, as has been mentioned before, 

 proved unsuccessful, or else could be given a different interpretation, as, for 

 instance, Kammerer's and Tower's results referred to above. Recently, how- 

 ever, some new observations of this kind have been carried out, certain ex- 

 perimentally acquired characters in animals under investigation having been 

 transmitted to the offspring along Mendelian lines, to this class belong, for 



