「共系」 - Living, Loving, Thinking, Again

大川真氏*1曰く、

おそらく皇位継承研究者で誰も言ってこなかったと思いますけど、古代の系譜論では、双⽅的bilateralではなく共系的cognaticで捉えたほうがより正確に把握できますよね。
— 大川真 (@okawa1220chuo) 2026年9月18日

「父系的な系譜表象とは別に、実際の皇統は父方・母方双方の血統が反復的に交差するcognaticな親族ネットワークを形成していた」
というのが学術的な表現となりますね。 https://t.co/phtcqkwdxM
— 大川真 (@okawa1220chuo) 2026年9月18日

Cognatic kinshipはWikipediaでは以下のように定義されている；

Cognatic kinship is a mode of descent calculated from an ancestor counted through any combination of male and female links, or a system of bilateral kinship where relations are traced through both a father and mother. Such relatives may be known as cognates.
https://en.wikipedia.org/wiki/Cognatic_kinship

EBSCO “Patterns of Descent and Inheritance”*2から少し切り取ってみる；

Patterns of descent and inheritance refer to the ways in which lineage and family ties are recognized and transmitted across generations within various cultures. Descent is typically categorized into unilineal and cognatic systems. Unilineal descent recognizes lineage through a single gender line, either matrilineal (through mothers) or patrilineal (through fathers), while cognatic descent encompasses both male and female lines, often resulting in more intricate familial relationships. Within cognatic descent are variations such as bilateral, where individuals belong to both parents' lineages, and ambilineal, allowing individuals to choose which lineage to follow.

Descent comprises socially recognized links between individuals in different generations within a family through several generations. Descent patterns or principles are used by cultures to limit the range of individuals through whom descent can be traced. There are a few general patterns of descent, including:
Unilineal,
Cognatic, and
Parallel.
Unilineal descent comprises a class of descent rules in which descent is traced through a single line (i.e., male or female). Although males and females are members of a unilineal family and are considered to be kin, descent links are only recognized through relatives of the same gender. Both matrilineal descent (in which descent is traced through the line of one's mother and female ancestors) and patrilineal descent (in which descent is traced through the line of one's father and male ancestors) are types of unilineal descent. However, approximately 40 percent of societies in the world trace descent through both the ancestors of one's father and mother to some degree. This is done through nonunilineal or cognatic descent principles—rules in which both male and female lines are used to determine the kinship relationships between individuals.
Cognatic descent patterns typically result in more complex and varied familial relationships than those resulting from unilineal descent rules. There are four variations in cognatic descent rules:
Ambilineal,
Bilineal,
Bilateral, and
Parallel.
In this pattern, descent is traced and kinship established through both the female and male lines. In bilineal descent, every individual is a member of both his/her mother's and father's lineage. In bilateral descent, another type of cognatic descent, descent is traced from all biological ancestors no matter the gender or whether they are in the mother's or father's lineage. In bilateral descent, all children regardless of gender are members of both their mother's and father's families. Bilateral descent is the most common pattern of descent in Europe and the Americas. A third type of cognatic descent is ambilineal descent. This is a pattern of descent in which descent is traced and kinship established through either the female or male line. In ambilineal descent, individuals may select one line (i.e., female or male) to trace descent. However, each generation can also choose the parent through which he or she will trace lineage. Therefore, a family line may be matrilineal in one generation and patrilineal in another generation. Ambilineal descent is not a common pattern of descent.
Finally, parallel descent is a form of unilineal descent patterns in which males trace their ancestry through the male line and females trace their ancestry through the female line of the family. Each individual is a member of only one descent group (i.e., male or female).

池田光穂氏*3の分類はちょっと違うようだ；

出自には大きく、単系出自と多系出自に分類できる。
1.　単系出自（unilineal descent）
1.1　父系出自（patrilineal descent）：全体の44%を占める（めやす）
1.2　母系出自（matrilineal descent）：　　　15%
1.3　双系出自（bilineal descent）／二重出自（double descent）
　　　　　母系［牛の相続］と父系［土地の相続］の帰属や継承が同時に存在する
1.4　平行出自（parallel descent）；男は父系、女は母系
1.5　選系出自（ambilineal descent）*4；各自が父系・母系を選択可能
2.　多系出自（multilineal descent）／双方的bilateral／共系的cognatic
　　　　自己より上の代にあるすべての個人を通じて出自をたどる
（「出自について」https://navymule9.sakura.ne.jp/descent_in_ethnology.html）

上掲の英語版Wikipediaの”Cognatic kinship”の項の日本語版が「女系」となっていて吃驚したが*5、「共系」という訳語はかなり一般的に使用されているようだ。
細かい区別（それも食い違う分類）に頭がくらくらするということもあるのだが、「父方・母方双方の血統が反復的に交差する」というのは、松木武彦『古墳時代の歴史』*6を読む限り、皇室に一元化される以前の王権の継承の在り方としては、納得できるものだ。