186 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the descendants." Crossing or recombination of genes by mating, of 

 individuals of unequal genotype is the only real cause of variability. 

 Mutation exclusively consists of an occasional loss of a gene without 

 visible cause. No matter where we find rats of the Rattus group 

 there are never more than one kind of tree-rat of this group, one 

 house-rat, and one field-rat simultaneously present in one locality. The 

 three kinds cross with facility and produce fertile hybrids ; colonies of 

 aberrant forms thus arise. In seaports new types are continually arising 

 by crossing with imported rats. There is no real antagonism between 

 Mus rattus and M. norvegicus, but these do not mate together, or, if they 

 do, no offspring result. 



Evolution is the result of a combination of all those causes which 

 heighten variability and which limit it. The only cause of inheritable 

 variability in Metazoa is amphimixis. All those causes that tend to 

 reduce the potential variabiHty of a group tend to make varieties or 

 species of these groups. Such causes are isolation, migration, adaptation, 

 selection, and especially the fact that either periodically or regularly 

 the number of individuals of one generation is very much smaller than 

 that of the preceding one. This cause of purification of the type occurs 

 everywhere, and operates quite regardless of adaptation or fitness. To 

 this cause, working upon variation, may be ascribed numerous charac- 

 teristics for which we can invent no use. for which species are never- 

 theless pure. 



Post-natal Growth of Kidney of Albino Rat.* — John A. Kittelson 

 has made a minutiose series of measurements and enumerations. The 

 post-natal growth of the cortex of the kidney is fairly uniform, show- 

 ing in comparison with the entire body a relative increase between birth 

 and two weeks, decreasing slightly thereafter. The growth of the 

 medulla is more varied. The volumetric ratio of medulla to cortex 

 changes greatly in the course of growth. 



The new-born rat has 10,465 fully-formed renal corpuscles, or 15,533 

 including those incompletely formed. The similar numbers at one week 

 are 19,682 and 26,598 ; at two weeks, 24,061 and 24,091. At three 

 weeks 25,930 were counted, at seven weeks 28,583, at twelve weeks 

 28,863 ; so that practically the total number is reached during the third 

 post-natal week. The average diameter increases from about 62 micra 

 in the new-born to about 127 in the adult. In the adult human kidney 

 there are about 1,040,000 renal corpuscles, 



"Proboscis Pores" in Craniate Vertebrates.f— Edwin S.Goodrich 

 gives an account of the complex minute structure of the epithelium 

 lining Hatschek's pit on the roof of the buccal cavity in Amphioxus, 

 and of the development of this pit and of the pre-oral pit from the 

 left anterior ccelomic sac and an ectodermal ingrowth respectively. 

 The pre-oral pit becomes the wheel-organ of the adult. The ciliated 

 cells of Hatschek's pit are of mesodermic origin, but the rod- bearing 



♦ Anat. Record, xiii. (1917) pp. 385-408. 



t Quart. Journ. Micr. Soc, Ixii. (1917) pp. 539-54 (1 pi. and 8 figs.). 



