DISOSE AND ITS EFFECTS. 45 
change of function, inasmuch as they are transformed 
from urinary to reproductive functions in the female, 
whilst in the male they are functionless. 
The Oviducts—The history of the excretory organs 
indicates that the kidneys were larger in the an- 
cestors of vertebrata than in existing forms, and 
the urinary excretion was conveyed by a series of 
ducts to the exterior of the body, whereas the kidney 
possesses now only one duct, the ureter. Of the once 
extensive renal system, the higher vertebrata possess 
relics in the form of the Wolffian bodies and their 
ducts, structures which are relatively very large in the 
embryo, but towards the mid-period of intra-uterine 
life dwindle, and are for the most part present in 
adults as vestiges. Although the glandular portions 
of the primitive renal system atrophy, and are per- 
manently replaced by the kidney, the ducts belonging 
to them undergo a great change, and become utilized 
for reproductive purposes. 
In some fish the ova are shed into the abdominal 
cavity from the ovary, and then escape to the exterior 
by small openings near the anus, known as genital 
pores. In many fish the ducts which belonged to the 
anterior section of the primitive kidney become modified 
into egg-conduits or oviducts. 
When dealing with the Laws of Variation, Darwin 
states that “a part developed in any species to an 
extraordinary degree or manner, in comparison with 
the same part in allied species, tends to be highly 
variable.” This law may be extended beyond the 
scope of species, and applied to the reproductive ducts, 
