We may first divide all the different phenomena of inheritance into two groups, which we may distinguish as the transmission of _inherited_ characters, and the transmission of _acquired_ characters; and we may call the former the _conservative_ transmission, and the latter the _progressive_ transmission by inheritance. This distinction depends upon the exceedingly important fact that the individuals of every species of animals and plants can transmit to their descendants, not only those qualities which they themselves have inherited from their ancestors, but also the peculiar, individual qualities which they have acquired during their own life. The latter are transmitted by progressive, the former by conservative inheritance. We have now first to examine the phenomena of _conservative inheritance_, that is, the transmission of such qualities as the organism has already received from its parents or ancestors. (Gen. Morph. ii. 180.) Among the phenomena of conservative inheritance we are first struck by that which is its most general law, and which we may term the _law of uninterrupted or continuous transmission_. It is so universal among the higher animals and plants, that the uninitiated might overestimate its action and consider it as the only normal law of transmission by inheritance. This law simply consists in the fact that among most species of animals and plants, every generation is, on the whole, like the preceding—that the parents are as like the grandparents as they are like the children. “Like produces like,” as is commonly said, but more accurately “similar things produce similar things.” For, in reality, the descendants of every organism are never absolutely equal in all points, but only similar in a greater or less degree. This law is so generally known, that I need not give any examples of it.