May 10, 1912] 
smaller individuals resulting in a pronounced 
reduction in variability. The growth factor 
may be in wild cultures supplemented by 
racial heterogeneity. 
b. The Existence of an Assortative Mating. 
—Pearl’s conclusion that there is a real as- 
sortative mating in Parameciwm is backed up 
in every point by Jennings’s more detailed 
study. Large individuals conjugate with 
large, small with small. Pearl’s five series 
gave positive values of r—.480 to r=.794. 
Jennings’s eight “wild” cultures of unknown 
racial composition all gave positive correla- 
tions of r= .245 to r=.507. 
In twelve lots of conjugants from “pure 
races” with correlations ranging from 7—=— 
193 to r—.507, there are ten positive and only 
two negative constants—both based on very 
small numbers and not statistically trust- 
worthy with regard to their probable errors. 
The average of the twelve is r—.251. The 
correlation after separation of the pairs, is 
demonstrated to be greater than that deter- 
mined on individuals which are united. 
c. The Causes of the Assortative Mating.— 
Unquestionably, therefore, there is a real as- 
sortative conjugation. ‘The redemonstration 
of the existence of a correlation between con- 
jugants and the proof that it is not merely 
the result of heterogeneous cultures with only 
certain constituents in conjugation at one 
time is followed by a magnificent experi- 
mental-statistical analysis of the possible 
underlying factors. Equalization during 
mating, change of size during union, differ- 
ential contraction due to killing fluid, en- 
vironmental heterogeneity in the culture, are 
subjected to observation, experiment, meas- 
urement and statistical analysis. The specific 
results interest primarily the protozoologist. 
The broad and well-established conclusion is 
that the chief source of the correlation is, as 
Pearl five years ago maintained in Biometrika, 
a real assortative mating (larger individuals 
tending to conjugate with larger, smaller 
with smaller) arising in the fact that individ- 
uals must be of a certain degree of similarity 
in sizé to “ fit.” 
SCIENCE 
741 
d. The Evolutionary Significance of 
(a)—-(c).—In connection with physiological 
differences resulting in different times of con- 
jugation demonstrated by Jennings, the as- 
sortative conjugation is clearly a factor of the 
highest importance in the physiological isola- 
tion of the diverse races of Paramecium. 
Apparently, the smaller size and lower 
variability of the conjugants have not a se- 
lective influence in maintaining the type of 
the population. The smaller extremes are, for 
the most part, younger than the conjugants, 
while the larger non-conjugants themselves 
conjugate after a few fissions. Moreover, the 
offspring of conjugants increase in size and in 
variability until they equal (or, in size ex- 
ceed) the non-conjugants. Ex-conjugant off- 
spring of individuals separated before com- 
pleting conjugation. 
Finally, the questions concerning the origin 
of heritable variations through conjugation 
within the “pure line,” inextricably bound 
up as they are with the problem of the signifi- 
cance of conjugation, are reserved for a later 
paper. We are told, however, that heritable 
variations do, though rarely, arise as the re- 
sult of conjugation within the “pure race.” 
Important as are the biological results of 
these two papers, their greatest value lies, not 
in concrete facts, but in their demonstration 
of the value of a widely applicable method. 
Let us hope they are only the beginning of the 
attack upon the many problems of morphol- 
ogy, physiology and genetics of the microor- 
ganisms which the biometric methods applied 
by Pearl and Jennings render possible. 
J. ArtHur Harris 
“HE PASSING OF THE SLIME-MOULDS ” 
On the first page of a late number of Sctr- 
ENCE, April 13, appears the heading of a brief 
note entitled “The Passing of the Slime- 
moulds,” by Professor C. HK. Bessey. One who 
reads no farther, haying in mind such titles 
as “The Passing of the Buffalo,” “The Pass- 
ing of the Chinese Empire,” etc., might pos- 
sibly infer that the slime-moulds were doomed 
