The diversity of life is one of the most striking aspects of our planet; hence knowing how many species inhabit Earth is among the most fundamental questions in science. Yet the answer to this question remains enigmatic, as efforts to sample the world's biodiversity to date have been limited and thus have precluded direct quantification of global species richness, and because indirect estimates rely on assumptions that have proven highly controversial. Here we show that the higher taxonomic classification of species (i.e., the assignment of species to phylum, class, order, family, and genus) follows a consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated. This approach was validated against well-known taxa, and when applied to all domains of life, it predicts ~8.7 million (±1.3 million SE) eukaryotic species globally, of which ~2.2 million (±0.18 million SE) are marine. In spite of 250 years of taxonomic classification and over 1.2 million species already catalogued in a central database, our results suggest that some 86% of existing species on Earth and 91% of species in the ocean still await description. Renewed interest in further exploration and taxonomy is required if this significant gap in our knowledge of life on Earth is to be closed.

Classification分类
Kingdom: 界
Most textbooks now list 6 kingdoms -- animal, plant, protists (amoebas and such), fungi, bacteria, and archaebacteria (also called extremophiles).
Phylum: 门
There are more than 30 phyla in the Animal Kingdom and 9 or 10 in the Plant Kingdom. Phylum Chordata is the one we're most familiar with -- it includes humans, birds, fish, and all other vertebrates (animals with a backbone). Phylum Arthropoda includes insects, spiders, lobsters, etc. Arthropods have segmented bodies with the segments grouped into two or three distinct sections. They have hard external skeletons, or exoskeletons, that are shed and regenerated as the animals grow.
Class: 类
The various phyla are divided into classes -- Phylum Chordata is divided into the classes: amphibians, birds, mammals, reptiles and fish.
Order: 顺序
Scientific groupings don't follow hard and fast rules. Once we get to the "order" of a living thing, there sometimes begins to be some disagreement about where it belongs. You may find that different sources group creatures in different orders or families. And you may find that a creature has its order or family changed as more information is learned.
Family: ，家族
The family is a relatively new scientific concept. It is a way scientists group similar genuses together. This is not the "mom, dad, brother and sister" type of family!
Genus: 属
Two or more species that share unique body structures or other characteristics are considered to be closely related and are placed together in a genus. Sometimes a genus might include only a single species if there is nothing else in the world that has similarities with it. The genus is the first part of the scientific name of a species. The genus is always spelled with a capital letter and in italics.
Species: 物种
A species can be defined as a group of individuals that breed together to produce fertile offspring. Individuals of a species cannot breed with other such groups. It is sometimes possible for different species to breed, but the offspring will be sterile. A mule is the sterile offspring of a donkey and a horse, and the mule can never mate and reproduce itself. The species is the second part of the scientific name of a species. The species is always spelled with a lower case letter and in italics.

How many animal classes are there?
88 classes
1. Insecta – insects
2. Entognatha – entognathids
3. Arachnida – arachnids
4. Diplopoda – millipedes
5. Chilopoda – centipedes
6. Pauropoda – pauropods
7. Symphyla – symphylans
8. Mammalia – mammals
9. Reptilia – reptiles
10. Amphibia – amphibians
11. Aves – birds
12. Bivalvia – bivalves
13. Cephalopoda – cephalopods
14. Gastropoda – snails and slugs
15. Scaphopoda – tusk shells
16. Polyplacophora – chitons
17. Aplacophora – aplacophorans
18. Monoplacophora – monoplacophorans
19. Anthozoa – sea anemones and corals
20. Cubozoa – box jellyfishes
21. Hydrozoa – hydrozoans
22. Scyphozoa – true jellyfishes
23. Polypodiozoa – one species Polypodium hydriforme
24. Asteroidea – starfish and sea stars
25. Echinoidea – sea urchins and sand dollars
26. Ophiuroidea – brittle stars
27. Holothuroidea – sea cucumbers
28. Crinoidea – feather stars and sea lilies
29. Pycnogonida – sea spiders
30. Branchiopoda – fairy shrimps & tadpole shrimps
31. Remipedia – remipedes
32. Cephalocarida – horse shrimps
33. Maxillopoda – barnacles and copepods
34. Ostracoda – seed shrimps
35. Malacostraca – crabs, lobsters, shrimps, krill, woodlice, mantis shrimps, etc
36. Staurozoa – stalked jellyfishes
37. Ascidiaca – ascidicans and sea squirts
38. Thaliacea – salps
39. Appendicularia – larvaceans
40. Chondrichthyes – cartilaginous fishes, sharks, rays
41. Actinopterygii - ray-finned fishes
42. Sarcopterygii – lobe-finned fishes
43. Myxini – hagfishes
44. Cephalaspidomorphi – lampreys
45. Eurotatoria – monogononts
46. Pararotatoria ( also called Digononta) – digononts
47. Gymnolaemata – class of bryozoans or moss animals
48. Phylactolaemata - freshwater bryozoans
49. Stenolaemata – marine bryozoans
50. Nuda – class of comb jellies
51. Tentaculata – class of comb jellies
52. Eutardigrada – eutardigrades
53. Heterotardigrada – heterotardigrades
54. Mesotardigrada – one species Thermozodium esakii
55. Calcarea – calcareous sponges
56. Hexactinellida – glass sponges
57. Demospongiae – demosponges
58. Trichoplacia – ones species Trichoplax adhaerens
59. Polychaeta – polychaetes
60. Clitellata – clitellates
61. Phascolomatidea - class of peanut worms
62. Sipunculidea – class of peanut worms
63. Cestoda – tapeworms
64. Trematoda – flukes
65. Monogenea – monoegenetic flukes
66. Turbellaria – free-living flatworms
67. Secernentea – class of nematodes or roundworms
68. Enoplea – class of roundworms
69. Chromadorea – class of nematodes
70. Anopla – class of ribbon worms
71. Enopla – class of proboscis worms
72. Gordioida – Gordian worms
73. Nectonematoidea -hosrehair worms
74. Archiacanthocephala – class of spiny-headed worms
75. Eoacanthocephala – class of spiny-headed worms
76. Palaeacanthocephala – ancient thornheads
77. Sagittoidea – arrow worms
78. Enteropneusta – acorn worms
79. Pterobranchia – pterobranchs
80. Halicryptomorpha – class of priapulid worms
81. Priapulimorpha – class of priapulid worms
82. Seticoronaria – class of priapulid worms
83. Myxosporea – class of myxozoans
84. Malacosporea – class of myxozoans
85. Lingulata – class of brachiopods
86. Rhynchonellata – class of brachiopods
87. Craniata – class of brachiopods
88. Eucycliophora – class in the phylum Cycliophora

There are about 30 different phyla of animals
动物大约有30个不同的"门"
---- "门" or "类" 可能是诺亚保存的动物 分类

《游子吟》第六章第6节
按照达尔文的自然选择思想，物种的变化是各种微小变化的累积，进化应该是连续不断的。但这种设想显然与实际情况不符。在自然界，各类生物之间都是有明显区别的。如果进化是连续的，生物分类将无法进行。现行的分类法就是根据各生物类群间差异的大小将它们分为门、纲、科、属、种等类的。这种分类单位不完全是分类学家主观的意念，也有一定的客观标准。比如说，关于“种”的生物学定义，其中一条便是，种间杂交不能产生后代或即使产生后代，后代却没有生殖能力。虽然在植物中有远缘杂交的实例，这一条在动物中似比较严格。比如，马和驴交配后可以生骡子，狮、虎杂交也可生子，但其子皆无生育能力。所以，马和驴，狮和虎属于不同生物种。现存生物类种间的明显区别与连续进化的学说是矛盾的。

话说species的定义其实一直没有个绝对的共识啊
无论如何, 地球的生物实在太丰富了, 叫人不得不赞叹造物主的奇妙。
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无论如何, 那麼多的生物读起分类学来一定会要吐血的, 是哪个混蛋这样分的啊啊啊啊啊(不对

耶和华神极其重视受造物应当各从其类的原则。
所以，物种的分类（暂不谈具体的分类标准）不但有客观依据，更有圣经依据。
利未记19：19
你们要守我的律例。
不可叫你的牲畜与异类配合/Do not mate different kind of animals.
……
《圣经启导本》注释：
不可异类配种是贯彻神的造物应各从其类的原则。希伯来人自铜器时代中期已知道选择交配的事，但此例严禁用异类的牲畜交配产生混血牛羊，以防以色列人模仿，吸取外邦宗教习俗，应用在自己的生活上，渗入本身信仰中。
引申思考：
如果耶和华神禁止以色列百姓进行牲畜的异类配合，要他们在生活中持守“各从其类”的原则，那么，耶和华神自己会不会用“宏观进化”的方式（尤其是在动物界，令一个物种进化为另一个物种）使生命布满大地呢？