There are, however, certain facts, which recent experimenters have brought to light, that seem to show that the segregation is not so complete as the law requires. For example, the so-called pure extracted forms may be found, when bred with other varieties, to have some latent characters. Thus Cuénot observed that extracted pure albino mice, that is to say, those derived from hybrid forms, did not all behave alike when paired with other mice. Those which had been bred from grey × white hybrids behaved, on being crossed, differently to those that had been bred from black × white hybrids; and further, those derived from yellow × white hybrids yielded yet other results on being intercrossed. Castle records similar phenomena in the case of guinea-pigs, and accordingly draws a distinction between recessive and latent characters. Recessive characters are those which disappear when they come into contact with a dominant character, but reappear whenever they are separated from the opposing dominant character. Latency is defined by Castle as “a condition of activity in which a normally dominant character may exist in a recessive individual or gamete.” The ordinary Mendelian pictures a unit character in a cross that obeys Mendel’s law, as follows:— D R, the dominant character only showing. It seems to us that each unit character should be represented as a double entity, thus D(D), the portion within the bracket being latent. The cross would appear to be represented by the formula D(R) R(D), since the union appears to take the form of the transfer of the dormant latent characters. Now an extracted pure recessive will, on this hypothesis, bear the formula R(D) R(D). When such recessives are crossed the two dormant portions will ordinarily change places, and never appear, so that these extracted recessives will, under ordinary circumstances, appear to be as pure as the true pure recessives, which are represented by the formula R(R) R(R). Now, suppose that, from some cause or other, it is possible for the latent D to change places with the visible R, it is obvious that the impure nature of the extracted and hitherto apparently pure recessives will become manifest. This seems to be what happens under certain circumstances to the extracted albino mice. They possess latent the character of their dominant ancestor. Unit Characters Mendelian phenomena force upon us the conclusion that organisms display a number of unit characters, each of which behaves in much the same way as a radicle does in chemistry, inasmuch as for one or more of these characters others can be substituted without interfering with the remaining unit characters. For example, it is possible to replace the chemical radicle NHࠣ by the radicle Naࠢ; _e.g._ (NHࠣ)ࠢSOࠤ (ammonium sulphate) may be transformed into NaࠢSOࠤ (sodium sulphate).