To Point I.  First of all you will please note on p.19 of the Origin that the process of development of the Punaluan family is presented as having taken place so gradually that even in this century marriages of brother and sister (of one mother) have taken place in the royal family of Hawaii. And throughout antiquity we find examples of marriages between brother and sister, e.g., among the Ptolemies. Secondly, we must here distinguish between brother and sister deriving from the side of the mother, or deriving only from the side of the father; adelphos, adelphse come from delphos, womb, and originally signified, therefore, only brother and sister on the side of the mother. The feeling had survived a long time from the time of the mother-right that the children of the same mother who have different fathers, are more closely related than the children of the same father who have different mothers. The Punaluan form of the family excludes only marriages between the first group, but by no means between the second who according to the existing notion are not even related (since mother-right rules). As far as I know, the cases of marriage between brother and sister in ancient Greece are restricted either to those individuals who have different mothers or to those about whom this is not known, and for whom, therefore, the possibility is not excluded; hence, they are absolutely not in contradiction to the Punaluan usage. You have overlooked the fact that between the time of the Punaluan family and the time of Greek monogamy there lies the jump from the matriarchate to the patriarchate, which alters matters considerably. According to Wachsmuth s Hellen. Altertumern, in the heroic age of Greece, there is no sign of any concern about the too close blood relationship of husband and wife, except for the relation of parent and child (III, p.156). Marriage with one s own sister was not disapproved of in Crete (ibid., p.170). The last also according to Strabo, Bk.X, for the moment however, I cannot find the passage because of the absence of chapter divisions. By one s own sister I understand, until there is proof to the contrary, sisters on the father s side. To Point II.  I qualify your first major proposition as follows: According to the materialistic conception of history, the production and reproduction of real life constitutes in the last instance the determining factor of history. Neither Marx nor I ever maintained more. Now when someone comes along and distorts this to mean that the economic factor is the sole determining factor, he is converting the former proposition into a meaningless, abstract and absurd phrase. The economic situation is the basis but the various factors of the superstructure the political forms of the class struggles and its results constitutions, etc., established by victorious classes after hard-won battles legal forms, and even the reflexes of all these real struggles in the brain of the participants, political, jural, philosophical theories, religious conceptions and their further development into systematic dogmas all these exercize an influence upon the course of historical struggles, and in many cases determine for the most part their form. There is a reciprocity between all these factors in which, finally, through the endless array of contingencies (i.e., of things and events whose inner connection with one another is so remote, or so incapable of proof, that we may neglect it, regarding it as nonexistent) the economic movement asserts itself as necessary. Were this not the case, the application of the history to any given historical period would be easier than the solution of a simple equation of the first degree. We ourselves make our own history, but, first of all, under very definite presuppositions and conditions. Among these are the economic, which are finally decisive. But there are also the political, etc. Yes, even the ghostly traditions, which haunt the minds of men play a role albeit not a decisive one. The Prussian state arose and developed also through historical, in the last instance, economic causes. One could hardly, however, assert without pedantry that among the many petty principalities of North Germany, just Brandenburg was determined by economic necessity and not by other factors also (before all, its involvement in virtue of its Prussian possessions, with Poland and therewith international political relations which were also decisive factors in the creation of the Austrian sovereign power) to become the great power in which was to be embodied the economic, linguistic and, since the Reformation, also the religious differences of North and South. It would be very hard to attempt to explain by economic causes, without making ourselves ridiculous, the existence of every petty German state of the past or present, or the origin of the shifting of consonants in High-German, which reinforced the differences that existed already in virtue of the geographical separating wall formed by the mountains from Sudeten to Taunus. Secondly, history is so made that the end-result always arises out of the conflict of many individual wills, in which every will is itself the product of a host of special conditions of life. Consequently there exist innumerable intersecting forces, an infinite group of parallelograms of forces which give rise to one resultant product the historical event. This again may itself be viewed as the product of a force acting as a whole without consciousness or volition. For what every individual wills separately is frustrated by what every one else wills and the general upshot is something which no one willed. And so the course of history has run along like a natural process; it also is subject essentially to the same laws of motion. But from the fact that the wills of individuals who desire what the constitution of their body as well as external circumstances, in the last instance economic (either personal or social) impel them to desire do not get what they wish, but fuse into an average or common resultant, from all that one has no right to conclude that they equal zero. On the contrary, every will contributes to the resultant and is in so far included within it. I should further like to beg of you to study the theory from its original sources and not at second hand. It is really much easier. Marx hardly wrote a thing in which this theory does not play a part. The Eighteenth Brumaire of Bonaparte is an especially remarkable example of its application. There are many relevant passages also in Capital. In addition, permit me to call your attention to my own writings, Herrn E. D hring s Umw lzung der Wissenschaft and L. Feuerbach und der Ausgang der klassischen deutschen Philosophie where I give the most comprehensive exposition of historical materialism which to my knowledge exists anywhere. Marx and I are partly responsible for the fact that at times our disciples have laid more weight upon the economic factor than belongs to it. We were compelled to emphasize this main principle in opposition; to our opponents who denied it, and there wasn t always time, place and occasion to do justice to the other factors in the reciprocal interaction. But just as soon as it was a matter of the presentation of an historical chapter, that is to say, of practical application, things became quite different; there, no error was possible. Unfortunately it is only too frequent that a person believes he has completely understood a new theory and is capable of applying it when he has taken over its fundamental ideas but it isn t always true. And from this reproach I cannot spare many of the recent Marxists . They have certainly turned out a rare kind of tommyrot. To Point I again. Yesterday (I am writing now on the 22nd of September), I found the following decisive passage, in Schoe-mann s Griechische Altert mer (Berlin, 1855, I, p.52), which completely confirms the view taken above: It is well known, however, that marriages between half-brothers or sisters of different mothers was not regarded as incest in late Greece. I hope that the appalling parenthetical expressions which, for brevity s sake, have slipped from my pen, won t frighten you off, and I remain. 
Letters: Marx-Engels Correspondence 1890