It would be impossible to give here any just idea of the vast amount of rigid scientific experimentation which has been carried on in this field, or the certainty of many of the results. Sex is really known, about as well as anything can be known, to arise from the chemical causes discussed above. That is, the endocrine explanation is the correct one. One of the most significant results of the transplantation experiments is the evidence that _each individual carries the fundamental bases for both sexes_. When Goodale changed a male bird into a female as to secondary characters and instincts by replacing one secretion with another, he was faced with the following problem: How can a single secretion be responsible for innumerable changes as to feather length, form and colouring, as to spurs, comb and almost an endless array of other details? To suppose that a secretion could be so complicated in its action as to determine each one of a thousand different items of structure, colour and behaviour would be preposterous. Besides, we know that some of these internal secretions are _not_ excessively complicated--for instance adrenalin (the suprarenal secretion) can be compounded in the laboratory. We may say that it cannot possibly be that the ovarian or testicular secretion is composed of enough different chemical substances to produce each different effect. There remains only the supposition that the female already possesses the genetic basis for becoming a male, and _vice versa_. This is in accord with the observed facts. In countless experiments it is shown that the transformed female becomes like the male of her own strain and brood--to state it simply, like the male she would have been if she had not been a female. If we think of this basis as single, then it must _exhibit_ itself in one way in the presence of the male secretions, in another way under the influence of the female secretions. In this way a very simple chemical agent in the secretion might account for the whole difference--merely causing a genetic basis already present to express itself in the one or the other manner. This may be illustrated by the familiar case of the crustacea _Artemia salina_ and _Artemia Milhausenii_. These are so unlike that they were long supposed to be different species; but it was later discovered that the genetic basis is exactly the same. One lives in 4 to 8% salt water, the other in 25% or over. If, however, the fresh-water variety is put in the saltier water with the salt-water variety, all develop exactly alike, into the salt-water kind. Likewise, if the salt-water variety is developed in fresh water, it assumes all the characteristics of the fresh-water kind. Thus the addition or subtraction of a single chemical agent--common salt--makes all the difference.