SUMMARY OF CURRENT RESEARCHES. 
515 
As a rulo, forms with a greater number of critical units included 
in the gestation period are born in a condition more advanced than 
those with fewer. Mere length of gestation does not affect this; for 
a mouse attains in 20 days a degree of development as advanced as a 
rabbit in 30, or as a dog in 62 days. 
The association of a greater degree of development at birth and 
an increased number of critical units embraced in gestation, is due to 
the fact that forms with gestation periods of several critical units, 
must, in their ancestral history, have passed through conditions in 
which the number was smaller. 
But the matter is complicated by the fact that in horse, sheep, pig, 
and man, the critical unit seems to be no longer the equivalent of one 
ovulation unit, but almost of two. The gestation length has been 
doubled without altering the grade of development at birtli, a change 
which is perhaps associated with a probable increase of size in the 
course of ancestral history. 
In all mammals, ovulation is either abortive or suppressed during 
gestation, for its normal occurrence would lead to abortion. The corpus 
luteum is probably connected with the suppression. Its commencing 
degeneration some little time before the end of the gestation, like its 
rapid atrophy when fertilisation has not taken place, allows of prepara- 
tion being made for a new ovulation. The approach of this ovulation 
is, in a reflex manner, the direct cause of birth. 
The critical period, multiples of this, and the ovulation periods, 
must very frequently be times of abortion in mammals. 
Menstruation is comparable to an abortion prior to a new ovulation. 
It is an abortion of a decidua prepared for an egg, which was given 
off subsequently to the preceding menstrual period, and has escaped 
fertilisation. It is comparable to an abortive birth at a former critical 
period. 
Lactation, gestation, the ovulation unit, and the critical unit, are all 
connected as expressions of the rhythm of reproduction in mammals. 
The basis of this rhythm is in the ovary. By ovulation the rhythm is 
proclaimed throughout the reproductive life of the female ; in gestation 
the same rhythm is maintained, but in a modified fashion ; and as the 
span of uterine life draws to a close, it again asserts itself, and induces 
birth. Thus harmony and law reign in the reproductive life of 
Mammalia. 
Our summary is very closely in the author’s own words ; but it must 
be understood that wo have been forced to leavo out the concrete argu- 
ments. We will quote the final sentence : “ The reign of law prevails in 
the infinitely little no less than in the immeasurably vast ; and that this 
should be, is probably not less momentous and vital for human existence, 
than that the law which moulds a tear should guide a planet in its 
course.” 
Orthogenesis.* — The late Prof. G. H. Tb. Eimer used this word to 
denote progressive development in a definite direction, and it is pro- 
* ‘Entstehung der Arten auf Grund von Vererben erworbener Eigensckaften 
nach den Gesetzen d. organischen Wachsens. II. Tkeil. Orthogenesis der Schmetter- 
linge, ein Beweis bestimmt gerichteter Entwickelung und Ohnmacht der natiirlichen 
Zuchtwahl bei der Artbildung. Zugleich eine Erwiderung an August Weismann. 
1898 2 n 
