Nov.-DEc,, 1919.] THE ORCHID REVIEW. 107 
law formulated for Pisum, a law which he recognised was not applicable 
to all hybrids. A few years later, Mendel conducted some experiments 
with Hieracium, and pointed out their quite different behaviour from those 
with Pisum, a character also found in Salix and various other plants. 
Mendel’s papers were lost sight of until 1900. Their recovery and sub- 
sequent amplification need not detain us further. It is at least interesting to 
have carried the subject back to the eighteenth century, and to have shown 
how much Mendel himself overlooked. 
So much for the history of the question. The various definitions that 
have been propounded must be left to take care of themselves, but we may 
attempt a short survey of the facts upon which they are based. Species 
reproduce themselves true from seed; Mendelians would add, because they 
are homozygous. Seed production is an adaptation for the preservation of 
the species, dating from remote times, and it is secured by the union of two 
reproductive cells, male and female, each containing all the elements 
necessary for the production of new individuals, similar to the parents, but 
without such union incapable of developing further. After uniting, they 
develop and mature. Under ordinary circumstances these reproductive 
cells are derived from individuals of the same species, and thus are alike in 
constitution, hence the phrase “‘ like produces like.” Owiag toa variety of 
reasons, however, the male reproductive cell may be derived from another 
species, and hence is, in some respects at least, of different constitution 
from the female cell, yet is capable of fertilising it, the result being the 
production of a hybrid seed, which on germinating bears evidence of its dual 
origin. There is no other distinction, and no control can be exercised 
beyond that of selecting which particular fertilising cell to use. 
Everything, then, depends upon the composition of the uniting 
reproductive cells, each of which, as we have already seen, contains all the 
elements necessary for the development of its own. species. This is where 
the Mendelian conception, whatever it may be, comes in. The number of 
these elements is entirely problematical. They may be few, or they may 
be very numerous. They cannot be seen or handled, and they can only be 
identified by their behaviour. Mendelism observes certain results, and 
proceeds to account for them. Two species are crossed, and the resulting 
hybrid differs from both in certain characters. As the hybrid is half 
derived from each parent, it follows that the new characters: have also 
arisen by minor contributions from each parent, the latter being termed 
factors, or unit characters, because they are supposed to separate again, 
unchanged, when the reproductive cells of the hybrid are formed. Each 
reproductive cell may contain one, two, OF many units, and each one is 
