"That the cell consists of more elementary units of organization, is indicated by _a priori_ evidence so cogent as to have driven many of the foremost leaders of biological thought into the belief that such units must exist, whether or not the microscope reveals them to view. The modern conception of ultra-cellular units, ranking between the molecule and the cell, was first definitely suggested by Bruecke in 1861. "This idea of ultra-cellular units is common to most morphologists and physiologists. We are compelled by the most stringent evidence to admit that the ultimate basis of living matter is not a single chemical substance, but a mixture of many substances that are self-perpetuating without their loss of specific character."[J] [J] For a fuller discussion of the notion of these hypothetical units of organic existence, see Weismann's Germinal Selection, (Open Court Publishing Co., Chicago, 1896), especially the foot note, page 230. Naegeli's _Laws of Evolution_ are also worth special notice. As stated in the body of _Abstammungslehre_ they are as follows: 1. Asexual reproductive cells which arise by division, remain united and become tissue cells. 2. Asexual reproductive cells which arise by budding, instead of separating, become cell branches or branched cell threads. 3. Reproductive cells which arise by free cell formation become bodies which form a part of the cell contents. 4. Parts of a plant which arise by differentiation lie side by side and form a body of web-like or tissue-like structure. 5. A definite and previously limited growth continues, or a definite formation of parts of an ontogeny which has previously been present but once, is repeated. (Ampliation.) 6. The parts of an ontogeny become dissimilar, since the functions which were previously united become differentiated and since new dissimilar functions are produced in the various parts. This differentiation is either one of space between the parts of the ontogeny that appear near each other, or one of time between those that are derived from each other. 7. Parts which have become dissimilar by differentiation undergo a reduction, in which the intermediate forms are suppressed and at last only the qualitatively dissimilar forms with qualitatively dissimilar functions remain. 8. The environment in which plants live operates in different ways, directly as a stimulus or indirectly as a felt necessity and by this means lends to their forms and activities a definite expression of time and place, and thus brings about different adaptations. These become permanent through heredity, but are again gradually lost if other adaptations supersede them. Laws 1 to 4 may be expressed as one--the law of combination: Similar parts that are wholly or partly separated have the tendency to unite more and more completely and intimately into one continuous tissue.