PREiSENT Pl^OliLEMS IN EVOLUTION AND HEREDITY. IVSl 



speiiiiatozoa were sharply (lirteicntiated.* (1) lie rei;ai (led the ovum 

 as a cell especially desigiieil as a storehouse of hereditary (;hanicteris- 

 ties, each characteristic being rei)reseiite(l by material i)articles of some 

 kind; thus hereditary characters were handed down by simple cell divi- 

 sion, each fertilized ovum j^i viuj>- rise to the body cells in which its hered 

 itary characters were manifested and to new ova in which these ciiarac- 

 ters were conserved for the next generation (this [ortion of Brooks's 

 Ilieoiy is very similar to (ralton's and Weismann's). (li) The body 

 (u'lls have the powcu' of throwing off ''gcmmules," but this is exercised 

 mainly or exclusively wlien its normal functions are disturbed, as in 

 nu'tatrophic exercise or under change of environment. (.")) These gem- 

 uuiles may enter the ovum, but tlu' spermato/oan is their nuiin center. 

 According to this view the female cell is rather conservative and the 

 male cell progressive; the union of these cells produces variability in 

 the offspring, exhibited es[)ecially in the regions of the offspring cor- 

 responding to the regions of faactional disturbance in the i)arent. 

 This hypothesis was well (considered, and while that feature of it which 

 distinguishes the male and female germ cells as different in kind has 

 been disproved, and the whole conception of gemmules is uow abandoned, 

 the fact still remains that we shall nevertheless be obliged to offer some 

 hypothesis to explain the facts disregarded by Weismann for which 

 Brooks provides in his theory of the causes of variation. 



L'. Continuity of germ cells. — Tlu' central idea here is an outgrowth 

 ol'our more m(jdern knowledge of embryogenesis and histogenesis, and 

 is therefore comparatively recent; it is that of a fundamental dis- 

 tinction between the "germ cells,"' as continuous and belonging to the 

 race, and the "body cells," as belonging to the individual. Weismann 

 has retiued and elaborated this idea, but it was not original with him. 



liichard Owen.t in L.S4!), naeck(^l,t iu 18()(), llauber,§ in 1S71>, in turn 

 dwelt upon tlu^ distinction which Dr. Jaeger, now of nninufacturing 

 fame, lirst clearly stated: 



"Through a gn^at series of generations the germinal proto|)lasm re- 

 tains its specilic i)roperties, dixiding in every reproduction into an 

 ontogenetic poition, out of which the individual is built np, and a 

 })hylogenetic portion, wliich is reserved to form the rei)r(>ductive 

 mateiial of the mature otfspring. This reservation of the phylogenetic 

 material I described as the continuity of the gi^.rni protoplasm. - - 



Encai)suled in tlie onti)geneti(; material the i)hylogenetic i)rotoplasm 

 is sheltered tVoiu extei iial influences, and retains its specilic and em- 

 bryonic characters.*' Tiie latter idea has, under Weismann, been 

 expanded into the theoi-y ol isolation of the genu cells. 



Cxalton introduced the term "stirj)" to express the sum total of 



" The Law of Heredilij, iss:5. 



t Seo Piirthenogenesis, in liis Aiintomii of rerlehratvs. 

 \ Genenlle Morpholonic, \ ol. ii, p. 170. 

 ^Zool. Anz., vol. IX, p. Itiii. 

 II. Mis. lU 22 



