EXTERNAL INFLUENCES AS CAUSES OF VARIATION 281 



physical factors decide whether or not their eggs develop par- 

 thenogenetically." 1 The consideration seems to be largely one 

 of change in osmotic pressure, some organisms requiring increase 

 and some decrease. Plant lice are parthenogenetic only at high 

 temperatures and when the host plant has plenty of water. " If 

 we lower the temperature or let the plant dry out, sexual repro- 

 duction occurs." l It seems to be decided that Artemia salina 

 living in brackish waters is parthenogenetic, while its nearest 

 fresh-water relative, Branchipus, is not. 2 



12. From the fact that the beginning of segmentation is com- 

 mon in unfertilized and untreated eggs of many forms, it seems 

 to follow that the effect of fertilization or of treatment is largely 

 to accelerate a process which is able to begin alone but which 

 proceeds so slowly as to be overtaken by destructive processes 

 and the death of the egg before an embryo can form. 



13. The introduction of a small amount of a catalytic sub- 

 stance at the critically proper time (at maturity) seems in most 

 cases necessary to a cell division sufficiently rapid to insure the 

 continuation of life. 



14. The function of the spermatozoon would seem therefore 

 to be twofold, first, to introduce such a catalytic substance, 

 and second, to convey hereditary material. 



No student can consider these fundamental matters and fail 

 to realize the profound effect of external influences upon in- 

 ternal activities, nor can he avoid the conclusion that we must 

 revise our ideas as to the relation even of inorganic chemistry 

 and physical forces to the processes of life. Much that we 

 have considered as morphological and peculiarly vital is, after 

 all, evidently due to the operations of ordinary chemical and 

 physical laws. This does not make the facts of variability 

 less significant, but it does show the extent to which living 

 organisms have become accustomed to their ordinary sur- 

 roundings. 



1 Loeb, Studies in General Physiology, Part II, p. 683. 



2 " Janosik has found segmentation in the unfertilized eggs of mammalians." 

 Loeb, Studies in General Physiology, Part II, p. 543. Loeb expresses the con- 

 viction that possibly "only the ions of the blood prevent the parthenogenetic 

 origin of embryos in mammalians," and that a change in their blood might be 

 followed by parthenogenetic development. 



