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It seems that there are a number of animal and plant names that can either be
used with or without 類 as a suffix. For example, in case of 'fish', I have
seen both 魚類 and 魚 being used to denote the respective animals. Another
example would be 'mammal', which I have seen both as 哺乳動物 and 哺乳類. Is there
any difference in meaning involved in these cases, and if yes, in what
situations would each be used? I have the impression that the 類 versions may
be used more in the context of biology, but I am not sure about that.
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question
word-choice usage
類 in animal and plant names
Generally speaking, yes, words like 魚 **類** , 人 **類** , and 哺乳 **類** sound
more technical and scientific than 魚, 人, or 哺乳動物.

魚【さかな】(和語) is the word we usually use when we want to say _fish_ in daily
life, for example at supermarkets, while 魚類 (漢語) is only used in the
biological context. I think the basic difference between 人 and 人類 is the same
as the difference between _person_ and _mankind_ / _human beings_.

--- (Maybe you can stop reading this now) ---

On the other hand, from the expert point of view, 類 is a rather non-scientific
word. The word 類 is _not_ used in the current [Japanese biological
classification
system](http://ja.wikipedia.org/wiki/%E7%94%9F%E7%89%A9%E3%81%AE%E5%88%86%E9%A1%9E).
There are only ドメイン (domain), 界【かい】 (kingdom), 門【もん】 (phylum), 綱【こう】 (class),
目【もく】 (order), 科【か】 (family), 族【ぞく】 (genus), 種【しゅ】 (species), and the minor
supra-/sub-categories of these. The most technically authoritative name for
哺乳【ほにゅう】 **類** (mammalians) is 哺乳 **綱【こう】**.

> じつは、「類」とは、正式な分類単位の名前ではありません。 **いわば、俗語です** 。何らかの共通点がある生物を、便宜上、まとめて呼ぶ名です。 ...
> 「類」は、綱だけを指すのではありません。門や目や科や属を指すこともあります。どの分類単位を指すのかは、決まっていません。  
>  ( <http://blog.zukan.net/blog/2009/08/12-812_1.php> )

Japanese "類" is a _customary_ and ambiguous term for grouping any kind of
organisms, at any level. It seems that certain words like
[哺乳類](http://ja.wikipedia.org/wiki/%E5%93%BA%E4%B9%B3%E9%A1%9E) or
[魚類](http://ja.wikipedia.org/wiki/%E9%AD%9A%E9%A1%9E) already have strict
(customary) definitions, and are safely used by experts.

But there are cases where using 類 is obviously troublesome. For example, the
biological classification of "dog" is as follows:

  * Class: Mammalia 哺乳綱
  * Order: Carnivora ネコ目
  * Family: Canidae イヌ科
  * Genus: Canis イヌ族
  * Species: Canis lupus タイリクオオカミ種
  * Subspecies: _Canis lupus familialis_ イエイヌ

The word "イヌ類" is not strictly defined in this system, and it might refer to
イヌ科 (includes foxes), or イヌ族 (includes wolves), or イエイヌ (scientific name of
what we usually call dogs, and there are of course many types of dogs).
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answer